A 32-bit OS cannot make calls with SMC64 IDs, while a 64-bit OS must
invoke some PSCI functions with SMC64 IDs.

This patch introduces and makes use of a new macro to choose the
appropriate IDs based on the register width of the OS, which will allow
32-bit callers to use the PSCI client code.
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Tested-by: Hanjun Guo <hanjun.guo@linaro.org>
Cc: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>