drm: mxsfb: DRM_MXSFB should depend on ARCH_MXS || ARCH_MXC

Freescale/NXP i.MX LCDIF and eLCDIF LCD controllers are only present on
Freescale/NXP i.MX SoCs.  Hence add a dependency on ARCH_MXS ||
ARCH_MXC, to prevent asking the user about this driver when configuring
a kernel without Freescale/NXP i.MX support.

Fixes: 45d59d70 ("drm: Add new driver for MXSFB controller")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Marek Vasut <marex@denx.de>
Signed-off-by: Marek Vasut <marex@denx.de>
Link: https://patchwork.freedesktop.org/patch/msgid/98e74779ca2bc575d91afff03369e86b080c01ac.1669046358.git.geert+renesas@glider.be