This follows the pattern of other peripherals (I2C, SPI) to specify the
pins using pin objects instead of a pair of GPIO port and pin number. It
makes it easier to customise the UART pins for a particular board.
Previously, SPI was configured by a board defining MICROPY_HW_ENABLE_SPIx
to 0 or 1. Now, the board should define MICROPY_HW_SPIx_SCK, MISO, MOSI
and NSS. This makes it the same as how I2C is configured.
This config option is for the USB OTG pin, pin A10. This is used on
some boards but not others. Eg PYBv3 uses PA10 for LED(2), so it
shouldn't be used for OTG ID (actually PA10 is multiplexed on this
board, but defaults to LED(2)).
Partially addresses issue #1059.
Pulled in and modified work done by mux/iabdalkader on cc3k driver, from
iabdalkader-cc3k-update branch. That branch was terribly messy and had
too many conflicts to merge neatly.
