Add an implementation for IGMP

This adds an implementation for trapping IGMP packets to the CPU
which will identify IGMPv1/2/3 packets, but handle only v3.
The implementation then inserts/updates/deletes L3 MC entries
in the L3 lookup table. The entries consist of an Ipv4
Destination IP (the IPv4 MC address), a Source IP (0.0.0.0) and
a Portmask. Note that this implementation is not VLAN aware,
as there is no hardware support in the device.

An alternative strategy is to control switching of the L2-MC packets
in which the IPv4-MC packets are transported (dst-MaC is
01:00:5e:xx:yy:zz, with xx:yy:zz corresponding to bits in the Ipv4-MC
address). This will allow to use VLAN-aware packet switching. While
code support is there for table insert/update/deletes, some further
L2 configuration is missing.

There is no support for IPv6 MC, yet.

rtl837x_regs.h

@@ -127,6 +127,8 @@
 #define RTL837x_L2_DATA_OUT_B	0x5cd0
 #define RTL837x_L2_DATA_OUT_C	0x5cd4
 #define RTL837x_TBL_DATA_IN_A	0x5cb8
+#define RTL837x_TBL_DATA_IN_B	0x5cbc
+#define RTL837x_TBL_DATA_IN_C	0x5cc0
 #define RTL837x_PVID_BASE_REG	0x4e1c
 
 #define RTL837x_L2_TBL_FLUSH_CTRL	0x53d4
@@ -186,6 +188,7 @@
 #define RTL837X_IGMP_PORT_CFG		0x52a0
 #define IGMP_MAX_GROUP			0x00ff0000
 #define IGMP_PROTOCOL_ENABLE		0x00007c00
+#define IGMP_TRAP			0x0000002a
 #define IGMP_FLOOD			0x00000015
 #define IGMP_ASIC			0x00000000
 #define RTL837X_IGMP_ROUTER_PORT	0x529c