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

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Signed-off-by: Arnd Bergmann <arnd@arndb.de>