ARM: mvebu: avoid clang -Wtautological-constant warning

Clang warns about the comparison when using a 32-bit phys_addr_t:

drivers/bus/mvebu-mbus.c:621:17: error: result of comparison of constant 4294967296 with expression of type 'phys_addr_t' (aka 'unsigned int') is always false [-Werror,-Wtautological-constant-out-of-range-compare]
                if (reg_start >= 0x100000000ULL)

Add a cast to shut up the warning.
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Link: https://lore.kernel.org/r/20210323131952.2835509-1-arnd@kernel.org'
Signed-off-by: Arnd Bergmann <arnd@arndb.de>

drivers/bus/mvebu-mbus.c

@@ -618,7 +618,7 @@ mvebu_mbus_find_bridge_hole(uint64_t *start, uint64_t *end)
 		 * This part of the memory is above 4 GB, so we don't
 		 * care for the MBus bridge hole.
 		 */
-		if (reg_start >= 0x100000000ULL)
+		if ((u64)reg_start >= 0x100000000ULL)
 			continue;
 
 		/*