smccc: define generic IDs for feature discovery

Defines function IDs ARM_SMCCC_ARCH_FEATURES used to query SMCCC feature
support, applicable from Arm SMCCC v1.1 specification.

Defines macro ARM_SMCCC_RET_NOT_SUPPORTED as generic return identifier
for when a SMCCC feature is not supported.

Signed-off-by: Etienne Carriere <etienne.carriere@linaro.org>