[ARM] Introduce common defterm for all ARM hosts

We have quite few boards sharing same serial port. Also, in
future we would like to have a unified arch_defterm_xxx()
functions which behaves based on serial port selected by DTB.

This patch unifies all board specific arch_defterm_xxx() into
one common defterm implementation.

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