aarch64: smccc: Decouple PSCI from SMCCC

The current code is assuming that the SMC/HVC helpers can only be used
by the PSCI driver. This is wrong because a mechanism to call into the
secure monitor should be made available regardless of using PSCI or not.

For example several SoCs relies on SMC calls to read/write e-fuses,
retrieve the chip ID, control power domains, etc...

This patch introduces a new CONFIG_HAS_ARM_SMCCC symbol to enable the
SMC/HVC helpers support and export that to drivers that require it.

Signed-off-by: Carlo Caione <ccaione@baylibre.com>