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>

xen/arch/arm/include/asm/processor.h

@@ -39,7 +39,7 @@
 #define MIDR_VARIANT(midr) \
     (((midr) & MIDR_VARIANT_MASK) >> MIDR_VARIANT_SHIFT)
 #define MIDR_IMPLEMENTOR_SHIFT  24
-#define MIDR_IMPLEMENTOR_MASK   (0xff << MIDR_IMPLEMENTOR_SHIFT)
+#define MIDR_IMPLEMENTOR_MASK   (0xffU << MIDR_IMPLEMENTOR_SHIFT)
 #define MIDR_IMPLEMENTOR(midr) \
     (((midr) & MIDR_IMPLEMENTOR_MASK) >> MIDR_IMPLEMENTOR_SHIFT)