riscv: dts: microchip: reduce the fic3 clock rate

For the v2022.09 release of the reference design, the fic3 clock rate
been reduced from 62.5 MHz to 50 MHz as it allows timing to be closed
significantly more quickly by customers who chose to build the
reference design themselves.

Signed-off-by: Conor Dooley <conor.dooley@microchip.com>