drm/bridge: imx: i.MX8 bridge drivers should depend on ARCH_MXC

The various Freescale i.MX8 display bridges are only present on
Freescale i.MX8 SoCs.  Hence add a dependency on ARCH_MXC, to prevent
asking the user about these drivers when configuring a kernel without
i.MX SoC support.

Fixes: e60c4354 ("drm/bridge: imx: Add LDB support for i.MX8qm")
Fixes: 3818715f ("drm/bridge: imx: Add LDB support for i.MX8qxp")
Fixes: 96988a52 ("drm/bridge: imx: Add i.MX8qxp pixel link to DPI support")
Fixes: 1ec17c26 ("drm/bridge: imx: Add i.MX8qm/qxp display pixel link support")
Fixes: 93e163a9 ("drm/bridge: imx: Add i.MX8qm/qxp pixel combiner support")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Liu Ying <victor.liu@nxp.com>
Reviewed-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Link: https://patchwork.freedesktop.org/patch/msgid/42c542b53a1c8027b23a045045fbb7b34479913d.1656072500.git.geert+renesas@glider.be