TFT_eSPI: Support for Raspberry Pi Pico added

The TFT_eSPI library now supports the "official" Arduino IDE RP2040 board package and I have added DMA capability for the Pico/RP2040 when using SPI displays.

The ILI9341 display operates reliably at 62.5MHz so frame rates up to ~43fps is possible with DMA:

The movement of the circles is smooth and better than the video indicates!