boards: efr32_radio: Fix CONFIG_SYS_CLOCK_HW_CYCLES_PER_SEC

The BRD4187C has a 39 MHz high frequency crystal oscillator and
runs at a 78 MHz operating frequency.
Fix the CONFIG_SYS_CLOCK_HW_CYCLES_PER_SEC setting, so it reflects
the actual SYSCLK frequency.

Signed-off-by: Markus Fuchs <markus.fuchs@ch.sauter-bc.com>