i2c: thunderx: ACPI support for clock settings

Add support for reading the system clock and the TWSI clock
frequency from ACPI DSDT.

TWSI clock was already covered by using device_property_read().

Signed-off-by: Jan Glauber <jglauber@cavium.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>