Add load capacitors to 16 MHz crystal (C14, C15)

The crystal has a specified load capacitance of 8 pF. The AT86RF230 has
an internal capacitor array that can be trimmed in 16 steps from 0 pF to
4.5 pF.

The crystal has a nominal tolerance of 15 ppm at 25 C plus a temperature
tolerance of 15 ppm. The following deviations from 1 MHz at CLKM were
measured with the CNTR board, roughly at 20-25 C:

Board		Error		Meas. accuracy	C14/C15		Trim
		(f, ppm)	(ppm, nom.)	(pF)		(pF)
---------------	---------------	---------------	------------	--------
2, "orange"	+268		 99.6		NC		0
		+266		 99.5		NC		0
		+266		 40.0		NC		0
		+148		 99.8		NC		3.0
		+147		 99.7		NC		3.0
		+147		 40.0		NC		3.0
		+111		100.0		NC		4.5
		+110		 99.8		NC		4.5
		- 13		 99.9		15		0
		- 14		100.0		15		0
		+ 11		 99.9		12		0
		+ 11		 99.6		12		0
(cleanup)	+ 14		100.0		12		0
		+ 14		100.0		12		0

1, "red"	+263		 99.7		NC		0
		+263		100.0		NC		0
		+107		 99.3		NC		4.5
		+108		100.0		NC		4.5
		+ 24		 99.8		10		0
		+ 24		100.0		10		0
		-  9		 99.7		10		4.5
		-  9		100.0		10		4.5
(cleanup)	+ 14		100.0		12		0
		+ 14		100.0		12		0

(Multiple measurements to assess CNTR performance and drift. Note that
CNTR measurements are repeatable within 1 ppm, so the nominal accuracy
appears to be far too pessimistic.)

These measurements suggest that, combined with parasitic capacitance,
load capacitors of 12 pF make the crystal roughly half the trim range
faster than 16 MHz.