serial: Unneeded ARCH dependencies are removed

This patch performs a small cleanup tty/Serial/Kconfig file by removing
unneeded ARCH dependencies. This dependencies already included in
board/type symbols.

Signed-off-by: Alexander Shiyan <shc_work@mail.ru>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>