powerpc/mpc5121: Add clock driver

Plugs into the generic powerpc clock driver in
arch/powerpc/kernel/clock.c

The following subset of clk_interface is implemented:
    clk_get, clk_put:  get clock via name, release clock
    clk_enable, clk_disable:  enable or disable clock
    clk_get_rate:  get clock rate in Hz
    clk_set_rate:  stubbed
    clk_round_rate:  stubbed
    clk_set_parent: NULL
    clk_get_parent: NULL

Signed-off-by: John Rigby <jrigby@freescale.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>