arm64: move DBG_MDSCR_* to asm/debug-monitors.h

arm64: move DBG_MDSCR_* to asm/debug-monitors.h

In order to be able to use the DBG_MDSCR_* macros from the KVM code,
move the relevant definitions to the obvious include file.

Also move the debug_el enum to a portion of the file that is guarded
by #ifndef __ASSEMBLY__ in order to use that file from assembly code.

Acked-by: Will Deacon <will.deacon@arm.com>
Reviewed-by: Anup Patel <anup.patel@linaro.org>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
(cherry picked from commit 51ba248164d0eeb8b4f94d405430c18a56c6ac9a)
Signed-off-by: Christoffer Dall <christoffer.dall@linaro.org>