Fix Linux RTC deviation

Created by: Steinegger

The default clock frequency is 50MHz as also noted by the device tree file.

The rtc is generated by https://github.com/pulp-platform/ariane/blob/ad70ce1f30dad539e5a365ffe71a02aaf20b397e/fpga/src/ariane_xilinx.sv#L441-L451 which, as noted, halves the clock frequency, i.e. to 25MHz.

The current device tree specifies the timebase-frequency to be 15MHz https://github.com/pulp-platform/ariane/blob/ad70ce1f30dad539e5a365ffe71a02aaf20b397e/fpga/src/bootrom/ariane.dts#L14

Using a build of ariane-sdk and running e.g. sleep 30 show that time based operations have about a 30 to 40% error to them, i.e. the sleep example will take only about 19 second.