clk: at91: clk-master: add 5th divisor for mck master

clk-master can have 5 divisors with a field width of 3 bits
on some products.

Change the mask and number of divisors accordingly.

Reported-by: Mihai Sain <mihai.sain@microchip.com>
Signed-off-by: Eugen Hristev <eugen.hristev@microchip.com>
Reviewed-by: Claudiu Beznea <claudiu.beznea@microchip.com>