soc/sifive/fu540: Implement uart_platform_refclk for UART divisor calculation

After changing clock from 33.33Mhz to 1Ghz the UART divisor needs to be
recalculated. Return correct tlck frequency in uart_platform_refclk.

Change-Id: I2291e4198cf466a8334211c6c46bc3268fc979a9
Signed-off-by: Philipp Hug <philipp@hug.cx>
Reviewed-on: https://review.coreboot.org/28584
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>
Reviewed-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
1 file changed