[SYCL] [FPGA] Add latency control feature to FPGA extension docs

[shuoniu-intel]

Latency control parameters will be passed to FPGA pipe and FPGA LSU member functions via Properties. Note that the name convention of Properties type is not finalized yet, and usage of Properties in these two extension documents can be revised once the decisions firm up.

One minor change in FPGALsu.md: removing the cl:: namespace according to the SYCL 2020 spec.

This PR is ported from #4917.

[gmlueck]

I noticed a couple of typos below.

We also had a meeting on Friday to resolve some final issues with the compile-time property API, and this will result in some small name changes to your properties. @rolandschulz has the AR to update the compile-time property extension specification, which will document these changes. It might be easier to wait for that before merging this PR, so that you don't need to update it again later.

I think the changes are:

• The struct that defines the property should have the suffix "_key" (e.g. latency_constraint_key).

• Enums like latency_constraint::type should be declared outside of the property struct and prefixed with the name of the property. For example:

  enum class latency_constraint_type {
    none,
    exact,
    max,
    min
  };
  

• There should be a namespace scope variable without the "_key" suffix that applications can use to create the properties object. For example:

  template<int Target, type Type, int Cycle>
  constexpr inline latency_constraint_key::value_t<Target, Type, Cycle> latency_constraint;
  

  Actually, the preferred style is normally to create one such variable for each enumerator:

  template<int Target, int Cycle>
  constexpr inline latency_constraint_key::value_t<Target, latency_constraint_type::none, Cycle> latency_constraint_none;
  
  template<int Target, int Cycle>
  constexpr inline latency_constraint_key::value_t<Target, latency_constraint_type::exact, Cycle> latency_constraint_exact;
  
  // Etc.
  

  I'm trying to decide if this is a good style in your case, though, since this property has two other integer parameters.

[shuoniu-intel]

@gmlueck In that case I'll wait for finalization of property API now.

[gmlueck]

bader requested a review from gmlueck 4 hours ago

I think I am waiting for @shuoniu-intel to update with that latest property API:

@gmlueck In that case I'll wait for finalization of property API now.

[shuoniu-intel]

Close this PR because we will deliver experimental version API for latency control in oneAPI 2022.2 release.
Will create another new PR for formal version API.