arm64: Add macro to compose a sysreg field value

arm64: Add macro to compose a sysreg field value

A common idiom is to compose a tupple (reg, field, val) into
a symbol matching an autogenerated definition.

Add a help performing the concatenation and replace it when
open-coded implementations exist.

Suggested-by: Oliver Upton <oliver.upton@linux.dev>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Link: https://lore.kernel.org/r/20240122181344.258974-2-maz@kernel.org
Signed-off-by: Oliver Upton <oliver.upton@linux.dev>