drivers: omap2: Kconfig: make FB_OMAP2_DSS_INIT depend on OF

Commit 7378f114 ("media: omap2: omapfb: allow building it with
COMPILE_TEST") broke compilation without CONFIG_OF selected.
  CC      drivers/video/fbdev/core/fbmem.o
drivers/video/fbdev/omap2/omapfb/dss/omapdss-boot-init.c: In function ‘omapdss_update_prop’:
drivers/video/fbdev/omap2/omapfb/dss/omapdss-boot-init.c:68:2: error: implicit declaration of function ‘of_update_property’; did you mean ‘of_get_property’? [-Werror=implicit-function-declaration]
  of_update_property(node, prop);
  ^~~~~~~~~~~~~~~~~~
  of_get_property
cc1: some warnings being treated as errors
scripts/Makefile.build:312: recipe for target 'drivers/video/fbdev/omap2/omapfb/dss/omapdss-boot-init.o' failed
make[7]: *** [drivers/video/fbdev/omap2/omapfb/dss/omapdss-boot-init.o] Error 1
scripts/Makefile.build:559: recipe for target 'drivers/video/fbdev/omap2/omapfb/dss' failed
make[6]: *** [drivers/video/fbdev/omap2/omapfb/dss] Error 2
make[6]: *** Waiting for unfinished jobs....

Add OF dependency in order to make all configurations work again.

of_update_property() has no inline stub, and that that could be added as
an alternative.
Signed-off-by: Anders Roxell <anders.roxell@linaro.org>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Tested-by: Randy Dunlap <rdunlap@infradead.org>
Acked-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>

drivers/video/fbdev/omap2/Kconfig

-if ARCH_OMAP2PLUS || COMPILE_TEST
+if OF && (ARCH_OMAP2PLUS || COMPILE_TEST)
 
 source "drivers/video/fbdev/omap2/omapfb/Kconfig"