overlays: Add "drm" parameter to pitft35-resistive

The "drm" parameter forces the use of the hx8357d DRM driver, when by
default the fb_hx8357d framebuffer driver will be used in preference.

See: https://forums.raspberrypi.com/viewtopic.php?t=330088



Signed-off-by: Phil Elwell <phil@raspberrypi.com>