libpayload: Use unsigned integer for PDG_MASK

The clang compiler warns about undefined behavior about shifting signed
integers.

Change-Id: Ic4ce64207393ec4a8d6b188b35e0f436342826de
Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/84007
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>

payloads/libpayload/arch/arm/virtual.c

@@ -71,7 +71,7 @@ typedef uint64_t pgd_t;
 #define SECTION_SHIFT	30
 #define BLOCK_SHIFT	21
 #define PAGE_SHIFT	12
-#define PGD_MASK	(~0 << PAGE_SHIFT)
+#define PGD_MASK	(~0u << PAGE_SHIFT)
 
 static pmd_t *ttb_buff = 0;
 static uintptr_t work_block;