MIPS: Alchemy: fix Au1000/Au1500 LRCLK calculation

The Au1000 and Au1500 calculate the LRCLK a bit differently than
newer models: a single bit in MEM_STCFG0 selects if pclk is divided
by 4 or 5.

Signed-off-by: Manuel Lauss <manuel.lauss@gmail.com>
Cc: Linux-MIPS <linux-mips@linux-mips.org>
Patchwork: https://patchwork.linux-mips.org/patch/9148/


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