Optimize MCUX shim drivers to reduce memory footprint

[zephyrbot]

Reported by Maureen Helm:

The MCUX UART, I2C, and SPI shim drivers make calls to CLOCK_GetFreq in order to calculate the baud rate. This function dynamically figures out the system/bus clock frequency by querying the MCG registers, and is therefore large in terms of code size - roughly 1 KB.

A possible fix is to implement a proper clock_control driver that abstracts whether the clock frequency is looked up from a static variable or calculated on the fly.

(Imported from Jira ZEP-1981)

[pabigot]

Seems to still be relevant, but if this is never going to be worked it'd be nice to close it.