Davinci: configurable pll divider mask

This patch allows socs to override the divider ratio mask by setting an
optional field (div_ratio_mask) in the pll_data structure.

Signed-off-by: Cyril Chemparathy <cyril@ti.com>
Tested-by: Sandeep Paulraj <s-paulraj@ti.com>
Signed-off-by: Kevin Hilman <khilman@deeprootsystems.com>