MIPS: Alchemy: usb: use clk framework

Add use of the common clock framework to set and enable the 48MHz
clock source for the onchip OHCI and UDC blocks.

Tested on a DB1500.  (Au1200 and Au1300 use an external 48MHz crystal).

Signed-off-by: Manuel Lauss <manuel.lauss@gmail.com>
Cc: Linux-MIPS <linux-mips@linux-mips.org>
Patchwork: https://patchwork.linux-mips.org/patch/7467/


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