arm: Move sev() and wfe() definitions to common Arm header file

The sev() and wfe() asm macros are currently defined only for
mach-exynos. As these are common Arm instructions, move them to the
common asm/system.h header file, for both Armv7 and Armv8, so they
can be used by other machines.

wfe may theoretically trigger a context switch if an interrupt occurs
so add a memory barrier to this call.

Signed-off-by: Peter Hoyes <Peter.Hoyes@arm.com>
Reviewed-by: Andre <Przywara&lt;andre.przywara@arm.com>