PXA: Fix warning in zipitz2

Configuring for zipitz2 board...
zipitz2.c: In function ‘board_mmc_init’:
zipitz2.c:85:2: warning: implicit declaration of function ‘pxa_mmc_register’ [-Wimplicit-function-declaration]

Signed-off-by: Marek Vasut <marek.vasut@gmail.com>

board/zipitz2/zipitz2.c

@@ -28,6 +28,7 @@
 #include <serial.h>
 #include <asm/arch/hardware.h>
 #include <asm/arch/pxa.h>
+#include <asm/arch/regs-mmc.h>
 #include <spi.h>
 #include <asm/io.h>