video: fbdev: add the dependency of broadsheetfb in Kconfig

broadsheetfb is a platform driver and it should not be used on x86.
It should be used only by single ARM PXA board so add the dependency
in Kconfig.

Signed-off-by: Jia-Ju Bai <baijiaju1990@gmail.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>