Add more documentation on vlans and trunking

doc/trunking.md

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+# Trunking
+
+The RTL827x allows to combine multiple ports to a single logical port
+(trunking). Up to 2 trunk groups can be defined.
+
+## Trunking control
+Two registers RTL837x_TRUNK_CTRL_A (0x4f38) and RTL837x_TRUNK_CTRL_B (0x4f3c)
+define the trunk groups. Each holds a bitmap of ports making up the trunk
+group.  
+
+## Trunking API
+The code currently provides the following functions:
+```
+void trunk_set(uint8_t group, uint16_t mask) __banked
+```
+
+# VLAN configuration on the Serial Console
+For testing the following commands are provided on the serial console:
+```
+trunk <TRUNK-ID> [p1] [p2]...
+  create or set a trunk group. Trunk-ID is 1 or 2. ports a physical ports
+
+trunk <VLAN-ID> d
+  deletes the trunk group
+```