[MIPS] Alchemy: kill useless time variables

Since the commit 91a2fcc8

 ([MIPS]
Consolidate all variants of MIPS cp0 timer interrupt handlers) removed the
Alchemy specific timer handler, 'r4k_offset' and 'r4k_cur' variables became
practically useless, so get rid of them at last, renaming cal_r4off()
function into calc_clock() and making it return CPU frequency. Also, make
'no_au1xxx_32khz' variable static...

Signed-off-by: Sergei Shtylyov <sshtylyov@ru.mvista.com>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>