Add documentation for mirroring

doc/mirroring.md

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+# Mirroring
+
+The RTL827x provides support to mirror packages from a number of ports to a mirroring
+ports. For each mirrored port it is possible to define whether received, transmitted
+or both types of packages are being mirrored.
+
+## Mirroring control
+Mirroring is enabled by setting RTL837x_MIRROR_CTRL(0x6048):
+
+```
+RTL837x_MIRROR_CTRL = port << 1 | 0x1
+```
+Ports are numbered 0-9, 9 being the CPU-port.
+Mirroring is stopped by writing 0 to this register.
+
+The mirrored ports are configured in RTL837x_MIRROR_CONF(0x604c):
+
+```
+RTL837x_MIRROR_CONF = RRRR TTTT
+RRRR: 16 bit mask for ports where received packets are mirrored
+TTTT: 16 bit mask for ports where transmitted packets are mirrored
+```
+
+## Mirroring API
+The code currently provides the following functions:
+```
+void port_mirror_set(register uint8_t port, __xdata uint16_t rx_pmask, __xdata uint16_t tx_pmask) __banked
+void port_mirror_del(void)
+```
+
+# Mirroring on the Serial Console
+For testing the following commands are provided on the serial console:
+```
+mirror <mirroring port> <P1>[r|t] [P2][r|t] ...
+mirror to port <mirroring port>, source ports are P1 with the givent packet types, P2 and so on
+
+mirror d
+  Deletes mirroring configuration
+```