MIPS: Remove usage of CEVT_R4K_LIB config option.

Manuel Lauss <manuel.lauss@gmail.com> writes:

I introduced it as a fallback because early revisions of Alchemy hardware
we shipped had a non-functional 32kHz timer and had to rely on the r4k
timer instead.  Previously the r4k timer was initialized regardless, but
it's useless with the "wait" instruction.

So long story short:   I need either the on-chip 32kHz timer OR the r4k
timer if the 32kHz one is unusable, but not both, and r4k timer is useless
when au1k_idle is in use.

The current in-kernel Alchemy boards all work with the 32kHz timer, so I'm
not against removing R4K_LIB symbols.

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>