mpc85xx: support for Freescale COM Express P2020

This adds support for the Freescale COM Express P2020 board. This board
is similar to the P1_P2_RDB, but has some extra (as well as missing)
peripherals.

Unlike all other mpc85xx boards, it uses a watchdog timeout to reset.
Using the HRESET_REQ register does not work.

This board has no NOR flash, and can only be booted via SD or SPI. This
procedure is documented in Freescale Document Number AN3659 "Booting
from On-Chip ROM (eSDHC or eSPI)." Some alternative documentation is
provided in Freescale Document Number P2020RM "P2020 QorIQ Integrated
Processor Reference Manual" (section 4.5).

Signed-off-by: Ira W. Snyder <iws@ovro.caltech.edu>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>