smccc: Make constants available to assembly

Move constants out of the C-only section of the header next to the other
constants that are available to assembly.

Signed-off-by: Andrew Scull <ascull@google.com>
Reviewed-by: Sudeep Holla <sudeep.holla@arm.com>
Acked-by: Will Deacon <will@kernel.org>
Link: https://lore.kernel.org/r/20200618145511.69203-1-ascull@google.com
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>