Add new function in SSD1351 to stop SPI Bus

[yoonghm]

SSD1351 makes use of SPI which, at the moment, only supports external interrupts (PIOx_IRQn). I made a small change to disable SPI within SPIClass.

I added the following into ssd1351.h, around line 1027, to disable SPI.

  void end() {
    SPI.end();
  }