serial: small Makefile reordering

Move 21285 entry down alongside other UART drivers to be more consistent
with the rest of the file. It is kept before 8250 though, to preserve the
existing link ordering between those two.

Signed-off-by: Nicolas Pitre <nico@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>