watchdog: rti_wdt: Add 10% safety margin to clock frequency

When running against RC_OSC_32k, the watchdog may suffer from running
faster than expected, expiring earlier. The Linux kernel adds a 10%
margin to the timeout calculation by slowing down the read clock rate
accordingly. Do the same here, also to have comparable preset values
for both drivers.

Along this, fix the name of the local var holding to frequency - in Hz,
not kHz.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Stefan Roese <sr@denx.de>