port: show how many MAC addresses each port has learned

The switch keeps a count of the dynamic L2 entries learned on every
port in L2_LRN_PORT_CONSTRT_CNT (0x53ac + 4 * port, 13 bits). `stat`
prints it in a new MACs column and /status.json carries it as "macs".

On a SWTGW218AS the count matched the L2 table port by port, static
entries are not included, and taking a port down drops it to 0.

rtl837x_regs.h

@@ -214,6 +214,8 @@
 #define RTL837x_L2_TBL_FLUSH_CNF	0x53dc
 #define RTL837X_L2_LRN_PORT_CONSTRAINT	0x5384
 #define RTL837X_L2_LRN_PORT_CONSTRT_ACT	0x4f80
+#define RTL837X_L2_LRN_PORT_CONSTRT_CNT	0x53ac
+#define L2_LRN_PORT_CNT_MASK		0x1fff
 #define	RTL8373_REG_MAC_L2_PORT_MAX_LEN	0x1250
 
 /*