Serial: ns16550: Add support for CONFIG_SYS_NS16550_IER macro

On some processors this ier register configuration is different
for ex. Marvell Armada100

This patch introduce CONFIG_SYS_NS16550_IER macro support to
unconditionally initialize this register.

Signed-off-by: Prafulla Wadaskar <prafulla@marvell.com>