serial: Kconfig: drop unknown symbol 'ARCH_IMX'

Commit 8c8fdbc9 ("[ARM] Remove arch-imx from build system") dropped
ARCH_IMX. So this last reference to ARCH_IMX has been an
(inconsequential) nop since v2.6.31. And because ARCH_MXC practically
implies ARM we can also drop the reference to the latter symbol.

Signed-off-by: Paul Bolle <pebolle@tiscali.nl>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>