The Freescale i.MX8MP LDB bridge is only present on Freescale i.MX8MP
SoCs. Hence add a dependency on ARCH_MXC, to prevent asking the user
about this driver when configuring a kernel without i.MX SoC support.
Fixes: 463db5c2ed4aed01 ("drm: bridge: ldb: Implement simple Freescale i.MX8MP LDB bridge")
Signed-off-by: Geert Uytterhoeven <geert+renesas@xxxxxxxxx>