realtek: enable basic config for cpufreq framework

realtek: enable basic config for cpufreq framework

A new clock driver makes more sense if it can be used from consumers
like cpufreq.  Before we enable the driver we must tell the config that
the RTL838X and RTL839X targets allow CPU frequency changing.

Even though these targets currently rely on the CPU's internal R4K
timer, MIPS_EXTERNAL_TIMER is selected to allow for CPU frequency change
testing. The Realtek timers, which are clocked by the Lexra bus, still
need to be supported and used in order to provide correct wall times
when reclocking the CPU.

Signed-off-by: Markus Stockhausen <markus.stockhausen@gmx.de>
[add paragraph about MIPS_EXTERNAL_TIMER to commit message]
Signed-off-by: Sander Vanheule <sander@svanheule.net>