arm64: irqflags: Fix clang build warnings

Clang complains when passing asm operands that are smaller than the
registers they are mapped to:

arch/arm64/include/asm/irqflags.h:50:10: warning: value size does not
	match register size specified by the constraint and modifier
	[-Wasm-operand-widths]
                : "r" (GIC_PRIO_IRQON)

Fix it by casting the affected input operands to a type of the correct
size.

Reported-by: Nathan Chancellor <natechancellor@gmail.com>
Tested-by: Nathan Chancellor <natechancellor@gmail.com>
Signed-off-by: Julien Thierry <julien.thierry@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>