Add description of the J25 and J27 UART headers of the Macchiatobin. They use
uart peripherals that the CP0 (J25) and CP1 (J27) provide.

Even though J25 and J27 are labeled as UART header, the pins on these headers
can be muxed for other purposes. But the UART functionality is useful when the
board is mounted in an ATX style enclosure, since the console UART is not
accessible through the microUSB at CON9.
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Gregory CLEMENT <gregory.clement@bootlin.com>