ip17xx: Add support for IP175D

ip17xx: Add support for IP175D

Add support for the IP175D chip. Since the register set is vastly different
from the previous models, we cannot not use the register number tables in struct
register_mappings (except for VLAN_DEFAULT_TAG_REG), so we supply a different
set of low-level functions.

Unlike with the previous models, we keep the VLAN setup in our state structure
instead of querying the hardware (it would be much harder in case of IP175D,
because the mapping between hardware and software state is not 1:1). Therefore,
get_flags() and get_state() are no-ops.

Signed-off-by: Martin Mares <mj@ucw.cz>
Signed-off-by: Patrick Horn <patrick.horn@gmail.com>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@21721 3c298f89-4303-0410-b956-a3cf2f4a3e73