clk: at91: sam9x60: fix programmable clock

clk: at91: sam9x60: fix programmable clock

The prescaler mask for sam9x60 must be 0xff (8 bits).
Being set to 0, means that we cannot set any prescaler, thus the
programmable clocks do not work (except the case with prescaler 0)
Set the mask accordingly in layout struct.

Fixes: 1a1aa1c9408e ("clk: at91: add sam9x60 pmc driver")
Signed-off-by: Eugen Hristev <eugen.hristev@microchip.com>
Link: https://lkml.kernel.org/r/1569321191-27606-1-git-send-email-eugen.hristev@microchip.com
Acked-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Acked-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>