dm: i2c: allow disabling driver model in SPL

At present if U-Boot proper uses driver model for I2C, then SPL has to
also. While this is desirable, it places a significant barrier to moving
to driver model in some cases. For example, with a space-constrained SPL
it may be necessary to enable CONFIG_SPL_OF_PLATDATA which involves
adjusting some drivers.

This patch introduces a separate Kconfig symbols for enabling DM_I2C and
DM_I2C_GPIO support in SPL.

This will also help to get away from dirty workarounds to
achieve non-DM I2C support for SPL, which is currently used in some
board header files like:

ifdef CONFIG_SPL_BUILD
undef CONFIG_DM_I2C
endif

Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Heiko Schocher <hs@denx.de>
Signed-off-by: Igor Opaniuk <igor.opaniuk@foundries.io>
Reviewed-by: Tom Rini <trini@konsulko.com>