Add driver code for DFRobot's 8.8-inch DSI interface touch screen.

[fary99]

[6by9]

Thanks for the patches, but a few things to sort.

• Kernel coding style mandates tabs (set to 8-spaces) rather than spaces for indentation. The checkpatch.pl script (in the scripts directory) allows quick checking for obvious coding style errors. Currently panel-dfrobot-dsi.c produces "43 errors, 194 warnings, 3 checks, 278 lines checked".
• Adding a driver should be one patch that includes the driver C file, Kconfig, and Makefile changes. Ideally MAINTAINERS should also be updated if the driver too.
• Adding an overlay should be one patch that includes the dtbo file, Makefile change, and documentation in README.
• What is the need for a new dfrobot_gt911 driver when drivers/input/touchscreen/goodix.c already supports GT911? It's already used by the HyperPixel4 and Waveshare panel overlays.
• Why is your overlay enabling i2c_arm, when your panel and touch controller are both connected to i2c_csi_dsi?
• Have you considered adding a dsi0 override for use on Pi5?
• Why do df_panel_enable and df_panel_disable set register 0x01, which is also set by df_panel_bl_update_status as the backlight intensity? You have assigned the backlight to the panel, so that should be handled automatically, and then be enabled at the currently configured brightness.