arm: Remove zmx25 board and ARCH_MX25

This board has not been converted to CONFIG_DM by the deadline.
Remove it.  As this is the last ARCH_MX25 platform, remove those
references as well.

Cc: Matthias Weisser <weisserm@arcor.de>
Cc: Stefano Babic <sbabic@denx.de>
Signed-off-by: Tom Rini <trini@konsulko.com>