clk: at91: clk-master: split master clock in pres and divider

Split master clock in 2 controlling block: one for prescaler one for
divider. This will allow referencing correctly the CPU clock and
master clock in device trees.

Reported-by: Eugen Hristev <eugen.hristev@microchip.com>
Fixes: a6486228 ("clk: at91: sam9x60: add support compatible with
CCF")
Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>