Update PLL range with measured values

[mole99]

This PR updates the PLL range in the Housekeeping SPI docs to match reality.

I measured the following frequencies on my MPW-8 chips:

Chip	Measurements		Calculated
	fmax / 4	fmin / 4	fmax	fmin
1	27.57	11.74	110.28	46.96
2	26.95	11.43	107.8	45.72
3	26.61	11.27	106.44	45.08
4	27.48	11.73	109.92	46.92
5	27.95	11.89	111.8	47.56
6	27.14	11.61	108.56	46.44
7	28.52	12.1	114.08	48.4
8	27.87	11.79	111.48	47.16
9	26.73	11.35	106.92	45.4
10	26.63	11.4	106.52	45.6

Since Tim measured 51.17 to 118MHz at 1.8V, we can assume a frequency range of 55 to 105 MHz.