port: report link role and negotiated pause per port

Two per-port MAC status registers that nothing in the firmware reads:
0x6408 says which end is master on a BASE-T link, 0x63FC and 0x6400 say
whether pause was negotiated in each direction. port <n> show prints
both at the end.

The read sits in rtl837x_port.c rather than phy_show(), because these
are MAC registers and phy_show() is skipped for SFP ports. Master/slave
keys off machine.is_sfp[] so it stays off the fibre ports, where the
notion does not apply.

Bit N of each register is chip port N, which is not how
RTL837X_REG_LINKS_STS indexes its bytes a few lines above in the same
header.

rtl837x_regs.h

@@ -83,6 +83,10 @@
 #define RTL837X_REG_LINKS	0x63f0
 #define RTL837X_REG_LINKS_89	0x63f4
 #define RTL837X_REG_LINKS_STS	0x63E8
+/* Per-port status bits: bit N is chip port N */
+#define RTL837X_MAC_TX_PAUSE_STS	0x63FC
+#define RTL837X_MAC_RX_PAUSE_STS	0x6400
+#define RTL837X_MAC_MSTR_SLV_STS	0x6408
 
 /* Each nibble encodes the link state of a port.
    Port 0 appears to be the CPU port