PCMCIA: add Kconfig control for building sa11xx_base.c

sa11xx_base.c is currently built when either PCMCIA_SA1100 or
PCMCIA_SA1111 are selected.  Let's move the logic into Kconfig.

Acked-by: Dominik Brodowski <linux@dominikbrodowski.net>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>