clk: at91: sam9x60: fix main rc oscillator frequency

Main RC oscillator frequency is 12MHz according to datasheet
(chapter 27.2).

Fixes: 01e2113d ("clk: at91: add sam9x60 pmc driver")
Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Reviewed-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/1595403506-8209-8-git-send-email-claudiu.beznea@microchip.com


Signed-off-by: Stephen Boyd <sboyd@kernel.org>