temp: read the on-chip thermal sensor

The chip has a pair of thermal meters and nothing was reading them. This adds a
`temp` command that prints the current reading next to the one taken at power-on.

The result register at 0x340 splits a 19 bit value across two fields: bits 18:3
live in the low 16 bits, bits 2:0 in bits 18:16, with the sign at bit 18, degrees
in 17:10 and a fraction below. The nice part of that layout is that the degrees
fall out of a shift of the low half, so the conversion stays 16 bit.

The meter comes out of reset latched: the register keeps whatever was measured
when the board powered up and then never moves. Four samples 40 s apart came back
identical down to the raw ADC, and 0x340 matched the power-on register at 0x350.
Clearing bit 2 of the control register at 0x338 during init is what makes it live.

Measured on an SWTGW218AS: 58.7 to 59.5 C while running, with the power-on
register still showing the 49.2 C it started from. For what it is worth, the
scale lines up three ways: the threshold register reads -40 and +125, the slope
from warm-up works out at 4450 LSB/C, and the slope between the two meters at
4388 LSB/C.

We only read the meter that is up after boot. The other sits in reset with no
coefficients, so it would just repeat its power-on value.

Costs 231 bytes in BANK2.

rtl837x_regs.h

@@ -9,6 +9,13 @@
 #define RESET_SOC_BIT			0
 #define RESET_NIC_BIT			2
 
+#define RTL837X_TM_CTRL2		0x0338
+#define TM_CTRL2_EN_LATCH		0x04
+#define RTL837X_TM_RESULT		0x0340
+#define RTL837X_TM_RESULT_POWERON	0x0350
+// Temperature: 19 bit value, bits 18:3 in the low 16 bits of the register,
+// bits 2:0 in bits 18:16. Bit 18 signed, bits 17:10 degrees C, bits 9:0 fraction.
+
 #define RTL837X_REG_HW_CONF		0x6040
 // Bits 4 & 5: CLOCK DIVIDER from 125MHz for Timer