clk: pistachio: Add sanity checks on PLL configuration

When setting the PLL rates, check that:

 - VCO is within range
 - PFD is within range
 - PLL is disabled when postdiv is changed
 - postdiv2 <= postdiv1

Reviewed-by: Andrew Bresticker <abrestic@chromium.org>
Signed-off-by: Kevin Cernekee <cernekee@chromium.org>
Signed-off-by: Ezequiel Garcia <ezequiel.garcia@imgtec.com>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>