arm64/sysreg: Add definition of ISR_EL1

Add a definition of ISR_EL1 as per DDI0487I.a. This register was not
previously defined in sysreg.h, no functional changes.

Acked-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20221208-arm64-isr-el1-v2-2-89f7073a1ca9@kernel.org


Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>