scs: Remove references to asm/scs.h from core code

asm/scs.h is no longer needed by the core code, so remove a redundant
header inclusion and update the stale Kconfig text.

Tested-by: Sami Tolvanen <samitolvanen@google.com>
Reviewed-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Will Deacon <will@kernel.org>