soc : arm : microchip_mec Move custom busy wait config to MEC1501 SoC

Configuration item custom busy wait is required for all MEC1501
series if its RTOS timer driver is enabled. We moved the selection
of the timer from board level to MEC1501 SoC level.
Frequency selection remains at the board level.

Signed-off-by: Scott Worley <scott.worley@microchip.com>