SMCCC: Introduce function to check SMCCC function availability

SMCCC: Introduce function to check SMCCC function availability

Currently, 'SMCCC_ARCH_FEATURES' SMC call handler unconditionally
returns 'SMC_OK' for 'SMCCC_ARCH_SOC_ID' function. This seems to
be not correct for the platform which doesn't implement soc-id
functionality i.e. functions to retrieve both soc-version and
soc-revision.
Hence introduced a platform function which will check whether SMCCC
feature is available for the platform.

Also, updated porting guide for the newly added platform function.

Change-Id: I389f0ef6b0837bb24c712aa995b7176117bc7961
Signed-off-by: Manish V Badarkhe <Manish.Badarkhe@arm.com>