Add define for screen I2C frequency

[oscgonfer]

This PR adds a define for bus frequency for screens that are I2C based.

The reason for adding this is because many I2C sensors can't work at speeds higher than 400kHz. Currently, we are reclocking the I2C bus, but only ESP32 variants can do so. For more details, see arduino/Arduino#11457

Currently, only ESP32 can getClock(), while all cores can setClock():

• https://github.com/sandeepmistry/arduino-nRF5/blob/master/libraries/Wire/Wire.h#L50
• https://github.com/earlephilhower/arduino-pico/blob/master/libraries/Wire/src/Wire.h#L60
• https://github.com/stm32duino/Arduino_Core_STM32/blob/main/libraries/Wire/src/Wire.h#L103

For cases when I2C speed is different to the ones defined by sensors (see defines in sensor classes), we need to reclock I2C and set it back to the previous desired speed. Only for cases where we can know it OR predefine the speed, we can do this. Currently, the screen library is setting the I2C clock speed, but we are using the default from the library itself (700kHz), which is hardcoded as default on the init function.

Ideally, with this PR, we can move a step forward towards having support for these sensors on devices with or without screens, no matter the variant.

However, some things are pending:

• It'd be ideal to know if a screen is detected, what type it is (I2C or not at least), and have a getter method for knowing its speed. I am tagging @HarukiToreda for ideas or suggestions.
• Once we know if a screen is detected, for non-ESP32 base variants, we still need to know what is the I2C speed without a screen. We can assume 100kHz, but I may be overlooking something.

I am setting this as a draft until we agree on the above.