xen/arm: Avoid overflow using MIDR_IMPLEMENTOR_MASK

xen/arm: Avoid overflow using MIDR_IMPLEMENTOR_MASK

Value of macro MIDR_IMPLEMENTOR_MASK exceeds the range of integer
and can lead to overflow. Currently there is no issue as it is used
in an expression implicitly casted to u32 in MIDR_IS_CPU_MODEL_RANGE.
To avoid possible problems, fix the macro.

Signed-off-by: Michal Orzel <michal.orzel@arm.com>
Link: https://lore.kernel.org/r/20220426070603.56031-1-michal.orzel@arm.com
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Acked-by: Julien Grall <jgrall@amazon.com>