dtoc: Handle 'reg' properties with unusual sizes

At present dtoc assumes that all 'reg' properties have both an address and
a size. For I2C devices we do not have this. Adjust dtoc to cope.

Reported-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Signed-off-by: Simon Glass <sjg@chromium.org>
Tested-by: Kever Yang <kever.yang@rock-chips.com>