serial: ucc_uart: use of_property_read_u32() in ucc_uart_probe()

For this to work correctly on little-endian hosts, don't access the
device-tree properties directly in native endianness, but use the
of_property_read_u32() helper.

Reviewed-by: Timur Tabi <timur@kernel.org>
Acked-by: Timur Tabi <timur@kernel.org>
Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Signed-off-by: Li Yang <leoyang.li@nxp.com>