Add modules_install target

In order to be able to install the modules in yocto we need the
modules_install target to be in the Makefile.

Signed-off-by: Fabien Parent <fparent@baylibre.com>
diff --git a/Makefile b/Makefile
index e8c5bba..3e90232 100644
--- a/Makefile
+++ b/Makefile
@@ -31,6 +31,9 @@
 	make -C $(KERNEL_SRC) M=$(PWD) $(SDIO_MOD_NAME).ko
 endif
 
+modules_install:
+	$(MAKE) -C $(KERNEL_SRC) M=$(PWD) modules_install
+
 clean:
 	make -C $(KERNEL_SRC) M=$(PWD) clean
 # Check coding style