Add Downloading section to note use of submodules.
Add Nordic nRF51 GPIO definition.
Add Nordic nRF51 UART definition.
Add nRF51 C headers.
Support arrays of registers, "array: <size>" YAML field.

This handles repetition of single register. There may be cases when
group of registers is repeated. Deal with it on demand (may need another
syntax though - also, on C level (enclosed structures)).
nrf51 gpio: Use array for PIN_CNF.