MIPS: Alchemy: au1xmmc: use clk framework

Use the clock framework to get the peripheral clock rate to
correctly set the MMC/SD bus clock divider.

Signed-off-by: Manuel Lauss <manuel.lauss@gmail.com>
Cc: Linux-MIPS <linux-mips@linux-mips.org>
Patchwork: https://patchwork.linux-mips.org/patch/7475/


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