Add get_erase_value() support

[offirko]

Description

Add QSPIF BD with support for get_erase_value()

Pull request type

[X] Fix
[ ] Refactor
[ ] Target update
[ ] Functionality change
[ ] Docs update
[ ] Test update
[ ] Breaking change

[offirko]

@davidsaada , @dannybenor , @cmonr , @0xc0170 - I've added support for get_erase_value() to QSPIF Block Device. The verified returned value after erase is 0xff (identical to SPIF) - I'd appreciate your review

[0xc0170]

/morph build

[mbed-ci]

Build : SUCCESS

Build number : 3556
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/8639/

Triggering tests

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

[mbed-ci]

Exporter Build : SUCCESS

Build number : 3164
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/8639/

[mbed-ci]

Test : SUCCESS

Build number : 3338
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/8639/3338