Refine platform features control

Allow a platform to report its supported features in more details.
The new features defined are:
* SBI_PLATFORM_HAS_PMP
* SBI_PLATFORM_HAS_SCOUNTEREN
* SBI_PLATFORM_HAS_MCOUNTEREN

In addition, define the macro SBI_PLATFORM_DEFAULT_FEATURES as the set
of features that are generally expected to be supported by a Linux
capable platform.

Operations touching the features controlled with these falgs are not
executed if the platform does not set the corresponding feature flags.

Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>