configs: tinker-rk3288 disable CONFIG_SPL_I2C_SUPPORT

The SPL for the Tinker Board has to fit into 32 KiB. Currently this limit
is exceeded.

CONFIG_SPL_I2C_SUPPORT is not needed to move to main U-Boot. So let's
disable it.

Suggested-by: David Wu <david.wu@rock-chips.com>
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: David Wu <david.wu@rock-chips.com>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>