drm: mxsfb: DRM_IMX_LCDIF should depend on ARCH_MXC

The Freescale/NXP i.MX LCDIFv3 LCD controller is only present on
Freescale/NXP i.MX SoCs.  Hence add a dependency on ARCH_MXC, to prevent
asking the user about this driver when configuring a kernel without
Freescale/NXP i.MX support.

Fixes: 9db35bb3 ("drm: lcdif: Add support for i.MX8MP LCDIF variant")
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/6103c1aa65a7888c12d351ae63f29850f29f42b9.1669046403.git.geert+renesas@glider.be