Move EL drop definition out of boot.S

PSCI needs to be able to drop cores to EL2 repeatedly, and it doesn't
make sense to always throw CPUs through the original boot path.

This patch changes the EL drop into a macro, and moves it to a common
file that can be used by different boot protocol / service
implementations. While doing so, the SPSR value used is split out to be
more legible.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>