[POWERPC] Add common clock setting routine mpc52xx_psc_set_clkdiv()

[POWERPC] Add common clock setting routine mpc52xx_psc_set_clkdiv()

PSC drivers should not access the CDM registers directly.  Instead provide
a common routine for setting the PSC clock parameters with the required
locking.

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>