[ARM] pxa: add basic support for Colibri PXA300 module

This patch add basic support for Toradex' Colibri PXA300 module.
Ethernet is enabled conditionally, depdending on CONFIG_AX88796.

Signed-off-by: Daniel Mack <daniel@caiaq.de>
Signed-off-by: Eric Miao <eric.miao@marvell.com>