i2c: mpc: unify obtaining the MPC8533/44 I2C clock prescaler w/ MPC8xxx

Commit 8ce795cb ("i2c: mpc: assign the correct prescaler from SVR")
introduced the common helper function for obtaining the actual clock
prescaler value for MPC85xx. However, getting the prescaler for MPC8544
which depends on the SEC frequency ratio on this platform, has been always
performed separately based on the corresponding Device Tree configuration.

Move special handling of MPC8544 into that common helper. Make it dependent
on the SoC version and not on Device Tree compatible node, as is the case
with all other SoCs. Handle MPC8533 the same way which is similar
to MPC8544 in this regard, according to AN2919 "Determining the I2C
Frequency Divider Ratio for SCL".

Signed-off-by: Arseny Solokha <asolokha@kb.kras.ru>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>