serial: add missing SERIAL_CORE dependencies

Two new drivers have been added since 3.14, the MEN 16z135 uart, and
the ARM semihosting console. Both are missing an explicit 'select
SERIAL_CORE', which can leads build errors when no other driver
selects the core, as found during ARM randconfig testing.
In case of the ARM semihosting console, we also have to select
SERIAL_CORE_CONSOLE.

This adds the missing 'select' statements.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>