pcmcia: at91_cf: remove platform data support

Board file support has been removed for at91rm9200 back in 2014, make the
CF driver DT only.

Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20200930184804.3127757-2-alexandre.belloni@bootlin.com