ixp4xx: base support for Arcom Vulcan

This patch adds some basic support for the Arcom Vulcan (ixp425 based).

Supported devices include:
- XR16L551 serial ports
- External watchdog
- Flash
- SRAM
- 1-wire id

Signed-off-by: Marc Zyngier <maz@misterjones.org>
Cc: Imre Kaloz <kaloz@openwrt.org>
Cc: Krzysztof Halasa <khc@pm.waw.pl>
Signed-off-by: Krzysztof Hałasa <khc@pm.waw.pl>