ns16550: change reg-shift property default to zero

Change reg-shift property default to zero. When the integer property
is missing, it should be taken as zero. This is consistent to Linux
drivers/tty/serial/of_serial.c.

The x86 and most powerpc use reg-shift of 0. Most others use reg-shift
of 2. While reg-shift of 1 is rarely used.

Signed-off-by: Thomas Chou <thomas@wytron.com.tw>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Mugunthan V N <mugunthanvnm@ti.com>
Acked-by: Simon Glass <sjg@chromium.org>