powerpc/85xx: TQM8548: add missing support for RTC and LM75

It adds the missing RTC node to tqm8548.dts and enables support for
I2C, DS1307 and LM75 in the default configuration.

Signed-off-by: Wolfgang Grandegger <wg@grandegger.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>