mips: select REALTEK_OTTO_TIMER for Realtek platforms

When MACH_REALTEK_RTL is selected automatically select the
REALTEK_OTTO_TIMER option in order to support the Realtek platforms that
require it.

Signed-off-by: Chris Packham <chris.packham@alliedtelesis.co.nz>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>