dtoverlays: Fix pitft[28|35] overlays for 6.1 driver change. (#5508)

The overlays configured both irq-gpio and an interrupts/
interrupt-parent configuration for the stmpe MFD device.

irq-gpio was reworked in 6.1 and has issues with the configuration
as provided. Removing it and using the interrupts/interrupt-parent
configuration works fine, so do that.

See: https://forums.raspberrypi.com/viewtopic.php?t=351394



Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>