serial: sc16is7xx: split into core and I2C/SPI parts (core)

Split the common code from sc16is7xx driver and move the I2C and SPI bus
parts into interface-specific source files.

sc16is7xx becomes the core functions which can support multiple bus
interfaces like I2C and SPI.

No functional changes intended.

Also simplify and improve Kconfig entries.
  - Capitalize SPI keyword for consistency
  - Display list of supported ICs each on a separate line (and aligned) to
    facilitate locating a specific IC model
  - Add Manufacturer name at start of description string
  - Add UART keyword to description string

Suggested-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Hugo Villeneuve <hvilleneuve@dimonoff.com>
Link: https://lore.kernel.org/r/20240409154253.3043822-4-hugo@hugovil.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>