Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Fix a typo: the RXD0 pin is PA18, not PA19.

Configure the peripheral pins for MCI devices. Eliminate the now-unused
at91_pio_rm9200.h.

Create common routines for configuring the serial ports and use them
on all the at91rm9200 boards.

For our at91rm9200 boards, register which subtype of SoC is on the
board. We'll use this later to control the differences between these
two variants' pins.