i2c: designware: Support use in SPL

Allow this driver to set up an IO address in SPL using an 'early-regs'
property. This allows SPL to use the I2C driver without having to enable
the full PCI stack.

Also split out ofdata_to_platdata in designware driver since this is more
correct, and more convenient for the new logic.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>