avr32: factor out cpu_mmc_init()

cpu_mmc_init() is required by the init sequence to have a working MMC interface
on avr32. This will not be included in the binary if we omit the avr32 board.c
when building the generic board.

Signed-off-by: Andreas Bießmann <andreas.devel@googlemail.com>
diff --git a/arch/avr32/cpu/Makefile b/arch/avr32/cpu/Makefile
index 00cede3..e111db3 100644
--- a/arch/avr32/cpu/Makefile
+++ b/arch/avr32/cpu/Makefile
@@ -16,5 +16,6 @@
 obj-y			+= interrupts.o
 obj-$(CONFIG_PORTMUX_PIO) += portmux-pio.o
 obj-$(CONFIG_PORTMUX_GPIO) += portmux-gpio.o
+obj-y += mmc.o
 
 obj-$(if $(filter at32ap700x,$(SOC)),y) += at32ap700x/