powerpc/mpc5xxx: Switch mpc5xxx_get_bus_frequency() to use fwnode

Switch mpc5xxx_get_bus_frequency() to use fwnode in order to help
cleaning up other parts of the kernel from OF specific code.

No functional change intended.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Chris Packham <chris.packham@alliedtelesis.co.nz> # for i2c-mpc
Acked-by: Wolfram Sang <wsa@kernel.org> # for the I2C part
Acked-by: Mark Brown <broonie@kernel.org>
Acked-by: Marc Kleine-Budde <mkl@pengutronix.de> # for mscan/mpc5xxx_can
Acked-by: Damien Le Moal <damien.lemoal@opensource.wdc.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220507100147.5802-2-andriy.shevchenko@linux.intel.com