drm/rockchip: cdn-dp-core: add MODULE_FIRMWARE macro

The CDP DP component of the rockchip GPU driver requires firmware
so define MODULE_FIRMWARE for rockchip/dptx.bin so the details
are available in the module info.

Signed-off-by: Peter Robinson <pbrobinson@gmail.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20210506092951.515813-1-pbrobinson@gmail.com