MIPS: Push ARCH_MIGHT_HAVE_PC_SERIO down to platform level

Maybe once upon a time the select of ARCH_MIGHT_HAVE_PC_SERIO used to
make sense.  These days MIPS platforms long have done away with i8042 or
PS/2 style keyboard and mouse ports and embedded systems probably never
had them anyway so push the select down to the level of individual
platforms.

Fixes: f2d0b0d5

 ("MIPS: ranchu: Add Ranchu as a new generic-based board")
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>
Signed-off-by: James Hogan <jhogan@kernel.org>