[feature-nrf528xx] Fix TRNG for NRF52

[marcuschangarm]

When multiple TRNG objects are initialized, destroying the first
object will turn the TRNG off for the other objects.

This fix adds a counter to ensure that only when the last object
is destroyed will it cause the TRNG to be disabled.

[0xc0170]

How is this different from #6116 ?

[marcuschangarm]

This uses the new driver in SDK 14.2 which works whether the SoftDevice is enabled or not. In #6116 a different path is taken if the SoftDevice is not enabled or not present.

[0xc0170]

@pan- Can you help to review this patch?

[0xc0170]

/morph build

[mbed-ci]

Build : SUCCESS

Build number : 1203
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/6128/

Triggering tests

/morph test
/morph uvisor-test
/morph export-build
/morph mbed2-build

[mbed-ci]

Exporter Build : SUCCESS

Build number : 875
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/6128/

[studavekar]

new changes were added since last build

/morph build

[mbed-ci]

Build : SUCCESS

Build number : 1206
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/6128/

Triggering tests

/morph test
/morph uvisor-test
/morph export-build
/morph mbed2-build

[mbed-ci]

Exporter Build : SUCCESS

Build number : 877
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/6128/

[mbed-ci]

Test : SUCCESS

Build number : 1005
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/6128/1005