drm/fsl-dcu: introduce kernel parameter to specify fbdev depth

Add a kernel parameter legancyfb_depth (like the i.MX drm driver)
to control the legancy fbdev depth. Default to the so far hard
coded depth of 24-bit. Currently changing the framebuffer depth
is not possible from user space when using the fbdev emulation
layer... This provides a rudimentary mechanism to change depth
without having to change kernel code.

Signed-off-by: Stefan Agner <stefan@agner.ch>