board: ti: am57xx-idk: Configure the CDCE913 clock synthesizer

AM57xx-IDK boards contain the CDCE913 clock synthesizer, and their
reset crystal capacitance load value of 10pF is wrong leading into
lost packets in certain networking tests. Add DT data for this
device, and probe it from the board file to program the crystal
capacitance load value to 0pF to avoid any problems.

Signed-off-by: Tero Kristo <t-kristo@ti.com>