Makefile: Move drivers/i2c/ into drivers/Makefile

This rule should not be in the top-level Makefile. Now that we have a
consistent set of I2C Kconfigs for U-Boot proper, SPL and TPL, we can move
it.

Make use of the existing SPL/TPL rule in drivers/Makefile instead.

Signed-off-by: Simon Glass <sjg@chromium.org>