fbdev: wm8505fb: select CONFIG_FB_IOMEM_FOPS

The fb_io_mmap() function is used in the file operations but
not enabled in all configurations unless FB_IOMEM_FOPS gets
selected:

ld.lld-20: error: undefined symbol: fb_io_mmap
   referenced by wm8505fb.c
   drivers/video/fbdev/wm8505fb.o:(wm8505fb_ops) in archive vmlinux.a

Fixes: 11754a50 ("fbdev/wm8505fb: Initialize fb_ops to fbdev I/O-memory helpers")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Helge Deller <deller@gmx.de>