MPC8308RDB: minimal support for devboard from Freescale

This patch provides support for MPC8308RDB development board from
Freescale with a minimal set of features:
 Dual UART is supported
 NOR flash is supported
 Both TSEC Ethernet controllers are supported
 PCI Express initialization is supported

The following features are enabled in configuration but not fully tested:
 I2C (used to get the board revision)
 I2C-connected RTC
 VSC7385 switch

There is one (hopefully) minor issue: on soft reset the board sometimes
resets twice. I've not managed to find the fix for this problem yet.
As a workaround instruction cache can be disabled.

Signed-off-by: Ilya Yanok <yanok@emcraft.com>
Signed-off-by: Kim Phillips <kim.phillips@freescale.com>