clk: at91: fix masterck name

The master clock is actually named masterck earlier in the driver. Having
"mck" in the parent list means that it can never be selected.

Fixes: 1eabdc2f ("clk: at91: add at91sam9x5 PMCs driver")
Fixes: a2038077 ("clk: at91: add sama5d2 PMC driver")
Fixes: 084b696b

 ("clk: at91: add sama5d4 pmc driver")
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Acked-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Cc: <stable@vger.kernel.org> # v4.20+
Signed-off-by: Stephen Boyd <sboyd@kernel.org>