pxa: move mmc drivers to drivers/mmc

introduce new macro CONFIG_PXA_MMC to activate it

Signed-off-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
diff --git a/cpu/pxa/Makefile b/cpu/pxa/Makefile
index 8b4367e..42903b2 100644
--- a/cpu/pxa/Makefile
+++ b/cpu/pxa/Makefile
@@ -26,7 +26,7 @@
 LIB	= $(obj)lib$(CPU).a
 
 START	= start.o
-COBJS	= serial.o interrupts.o cpu.o i2c.o pxafb.o mmc.o usb.o
+COBJS	= serial.o interrupts.o cpu.o i2c.o pxafb.o usb.o
 
 SRCS	:= $(START:.o=.S) $(SOBJS:.o=.S) $(COBJS:.o=.c)
 OBJS	:= $(addprefix $(obj),$(SOBJS) $(COBJS))