MIPS: Reimplement get_cycles().

This essentially reverts commit efb9ca08b5a2374b29938cdcab417ce4feb14b54
(kernel.org) / 58020a106879a8b372068741c81f0015c9b0b96dbv [[MIPS] Change
get_cycles to always return 0.]

Most users of get_cycles() invoke it as a timing interface.  That's why
in modern kernels it was never very much missed for.  /dev/random however
uses get_cycles() in the how the jitter in the interrupt timing contains
some useful entropy.

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