clk: microchip: Initialize SOSC clock rate for PIC32MZDA.

Optional SOSC is an external fixed clock running at 32768HZ.
So Initialize SOSC rate as per PIC32MZDA datasheet.

Signed-off-by: Purna Chandra Mandal <purna.mandal@microchip.com>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>