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_init.c

@@ -92,8 +92,18 @@ void static sds_init(void)
 }
 
 
+void static tm_init(void)
+{
+	reg_read_m(RTL837X_TM_CTRL2);
+	sfr_mask_data(0, TM_CTRL2_EN_LATCH, 0);
+	reg_write_m(RTL837X_TM_CTRL2);
+}
+
+
 void rtl8373_init(void) __banked
 {
+	tm_init();
+
 	print_string("\nrtl8373_init called\n");
 
 	// r65d8:3ffbedff R65d8-3ffbedff
@@ -191,6 +201,8 @@ void rtl8373_init(void) __banked
 
 void rtl8372_init(void) __banked
 {
+	tm_init();
+
 	print_string("\nrtl8372_init called\n");
 
 	sds_init();