serial: s5p: Use dev_read_addr_ptr() to get base address

As the address read from device tree is being cast to a pointer, it's
better to use dev_read_addr_ptr() API for getting that address. The more
detailed explanation can be found in commit a12a73b6 ("drivers: use
dev_read_addr_ptr when cast to pointer").

Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org>
Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>