[ARM] pxa: fix frequency scaling for pcmcia/pxa2xx_base

The MCxx values must be based off memory clock, not CPU core clock.

This also fixes the bug where on some machines the LCD went crazy
while using PCMCIA.

Signed-off-by: Marek Vasut <marek.vasut@gmail.com>
Cc: Nicolas Pitre <nico@fluxnic.net>
Reviewed-by: Robert Jarzmik <robert.jarzmik@free.fr>
Signed-off-by: Eric Miao <eric.y.miao@gmail.com>