of: fix a build error to of_graph_get_endpoint_by_regs function

This patch fixes the below build error reported by Stephen,

     Stephen reported:
     After merging the drm-exynos tree, today's linux-next build (x86_64
     allmodconfig) failed like this:

     drivers/media/i2c/adv7604.o: In function `of_graph_get_endpoint_by_regs':
     adv7604.c:(.text+0x586c): multiple definition of `of_graph_get_endpoint_by_regs'
     drivers/media/i2c/adv7343.o:adv7343.c:(.text+0xa13): first defined here
     drivers/media/platform/soc_camera/atmel-isi.o: In function `of_graph_get_endpoint_by_regs':
     atmel-isi.c:(.text+0x1ec9): multiple definition of `of_graph_get_endpoint_by_regs'
     drivers/media/platform/soc_camera/soc_camera.o:soc_camera.c:(.text+0x2ce3): first defined here
     drivers/media/platform/soc_camera/rcar_vin.o: In function `of_graph_get_endpoint_by_regs':
     rcar_vin.c:(.text+0x307c): multiple definition of `of_graph_get_endpoint_by_regs'
     drivers/media/platform/soc_camera/soc_camera.o:soc_camera.c:(.text+0x2ce3): first defined here

     Caused by commit:
       a0f7001c18ca ("of: add helper for getting endpoint node of specific identifiers")

To fix the error, this patch declares of_graph_get_endpoint_by_regs function
with "static inline".

Signed-off-by: Inki Dae <inki.dae@samsung.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>