drm: bridge: DRM_FSL_LDB should depend on ARCH_MXC

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: 463db5c2

 ("drm: bridge: ldb: Implement simple Freescale i.MX8MP LDB bridge")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Marek Vasut <marex@denx.de>
Signed-off-by: Robert Foss <robert.foss@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/449e08ca791a3ca308de5477c1bdc1f6eb1b34e7.1652104211.git.geert+renesas@glider.be