MIPS: csrc-r4k: Refine rating computation

Increase frequency addend dividend to 10000000 (10MHz) to
reasonably accommodate multi GHz level mips_hpt_frequency.

Cap rating of csrc-r4k into 299 to ensure it doesn't go into
"Desired" range, given all the drama we have with CP0 count
registers (SMP sync, behaviour on wait etc).

Signed-off-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>