[DRIVERS] serial: Minor improvement to 8250 UART driver

Generally, in linux drivers folks use "reg-shift" instead of "reg-align"
to specify register alignment boundary in device tree.

This patch update 8250 UART driver to use "reg-shift" instead of "reg-align"
and "reg-io-width" to specify register width.

Signed-off-by: Anup Patel <anup@brainfault.org>