uqmi: support split-APN IPv4 and IPv6 dual-stack

uqmi: support split-APN IPv4 and IPv6 dual-stack

Add two new "v6apn" and "v6profile" properties, to support split-APN
dual-stack onfiguration. This extends the existing ipv4v6 PDP type,
allowing simultaneous connection to two distinct APNs,
one for IPv4 and one for IPv6.
The parameters override existing 'apn' and 'profile' respectively,
if set, but only for IPv6 part of the connection.
If unset, they default to their original values, constituting a standard
IPv4v6 setup.

If a different APN is set for IPv6, a corresponding profile MUST also be
configured, with a different ID, than the IPv4 profile, for example,
profile 2.
Both APNs must match ones configured through QMI or through 'AT+CGDCONT'
command.

Example configuration in UCI:

config interface 'wan'
        option proto 'qmi'
        option device '/dev/cdc-wdm0'
        option autoconnect '1'
        option pdptype 'ipv4v6'
        option apn 'internet'
        option v6apn 'internetipv6'
	option profile '1'
	option v6profile '2'

Corresponding profile configuration:
AT+CGDCONT?
+CGDCONT: 1,"IP","internet","0.0.0.0",0,0,0,0
+CGDCONT: 2,"IPV6","internetipv6","0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0",0,0,0,0

Signed-off-by: Lech Perczak <lech.perczak@gmail.com>