i2c driver for cc13xx/cc26xx is configured with incorrect frequency

[vanti]

Describe the bug
The I2C peripheral should be configured using the CPU clock frequency and
not the system clock frequency. This used to be fine because they were the
same before #19232, but now that the system clock is RTC-based (which
has a different frequency), we can no longer make that assumption.

To Reproduce
Steps to reproduce the behavior:
Create the file boards/shields/boostxl_sensors/boostxl_sensors.overlay:

&i2c0 {

        bme280@77 {
                compatible = "bosch,bme280";
                reg = <0x77>;
                label = "BME280";
        };

};

1. cd samples/sensor/bme280
2. west build -b cc1352r1_launchxl -- -DSHIELD=boostxl_sensors
3. west flash
4. See error

***** Booting Zephyr OS build zephyr-v2.0.0-1913-g30fc72a50e1b *****
Could not get BME280 device

Expected behavior
I would expect the sample to run.

Impact
Prevents users from using the I2C on this platform.

Environment (please complete the following information):

• OS: Ubuntu 18.04
• Toolchain: Zephyr SDK 0.10.3
• Commit SHA or Version used: 30fc72a