According to specs, the display can handle up to 10 MHz SPI traffic
(minimum cycle time is 100 ns). This patch sets the SPI port to
7.5 MHz (60 MHz clock with divisor 8). In my tests the display works
even fine with SPI frequency at 60 MHz. I also minimized the delays
(the spec doesn't require a long setup time) and use the registers to
check when transfer is done instead of using a fixed delay.
This patch also changes the swipe function to swipe pixel by pixel.
Otherwise, the swipe would happen too fast to be seen.
This changes the display update time from 11 ms to 1.5 ms.
