board: gateworks: gw_ventana: use comomn GSC driver

Use the common GSC driver.

This allows us to do some additional cleanup:
- use the GSC driver functions
- move waiting for the EEPROM to the SPL int (it will always be ready
after this)
- move eeprom functions into eeprom file and elimate GSC_I2C_BUS
- eliminate some redundant EEPROM reads (the EEPROM must be read in
SPL before relocation, in SPL after relocation, and in U-Boot init.
All subsequent uses can use the global structure)
- remove unnecessary header files and alphabatize includes

Signed-off-by: Tim Harvey <tharvey@gateworks.com>