MIPS: Alchemy: pci: use clk framework to enable PCI clock

Use the clock framework to get at the PCI clock source and enable
it on driver initialization.

Signed-off-by: Manuel Lauss <manuel.lauss@gmail.com>
Cc: Linux-MIPS <linux-mips@linux-mips.org>
Patchwork: https://patchwork.linux-mips.org/patch/7471/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>