i2c: Create a new Kconfig for I2C

At present we have CONFIG_SPL_I2C but not CONFIG_I2C. The reason
CONFIG_I2C is not strictly necessary is that:

a) We have CONFIG_SYS_I2C_LEGACY and CONFIG_DM_I2C for the two possible
   i2c stacks
b) In U-Boot proper, we always build drivers/i2c/ regardless of the
   options

Still, it is better to have CONFIG_I2C - it makes U-Boot proper similar to
SPL/TPL, so we can (in a future commit) simplify the Makefile rules.

Enable it by default, since as above, we have separate options
(SYS_I2C_LEGACY and DM_I2C) to control whether it is 'really' enabled.

Once we have migrated I2C to driver model, we can drop SYS_I2C_LEGACY and
make DM_I2C become I2C. For now, this lets us simplify the Makefile rules.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Heiko Schocher <hs@denx.de>