clk: at91: sam9x60-pll: add support for HW PLL freq dividers

clk: at91: sam9x60-pll: add support for HW PLL freq dividers

Add support for hardware dividers for PLL IDs.In sam9x7 SoC,
PLL_ID_PLLA and PLL_ID_PLLA_DIV2 has /2 hardware dividers
each.

fcorepllack -----> HW Div = 2 -+--> fpllack
                               |
                               +--> HW Div = 2 ---> fplladiv2ck

Signed-off-by: Varshini Rajendran <varshini.rajendran@microchip.com>