serial: s5p: Use named constants for register values

Get rid of magic numbers in s5p_serial_init() when writing to UART
registers. While at it, use BIT() macro for existing constants when
appropriate.

No functional change.

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>