The touchscreen is the SPI device number 3, so it uses the 0x10142XXX SPI registers. It is initialized by issuing the following SPI commands:

spi_select_reg(3, 0x67);
spi_offset_mask(3, 0x26, 0x80, 0x80);
spi_select_reg(3, 0x67);
spi_offset_mask(3, 0x24, 0, 0x80);
spi_select_reg(3, 0x67);
spi_offset_mask(3, 0x25, 0x10, 0x3C);

Once the touchscreen is initialized, you can start polling touch data:

u8 raw_touchdata[0x40];
spi_select_reg(3, 0x67);
spi_read_offset(3, 0x26); //The return value of this is checked against "& 2"
spi_select_reg(3, 0xFB);
spi_read_offset_array(3, 1, raw_touchdata, 0x34);

This is the format of the touchdata report:

When the touchscreen is not touched, all the coordinates report 0xFFFF, and since touch coordinates have only 12 bits, you can check if the touchscreen is pressed by checking the 4th bit of the MSB. For example: pendown = !(raw_touchdata[0] & BIT(4))