The same functionality was added to Linux for i2c bus registration with this
commit message:

"
of: base: add function to get highest id of an alias stem

I2C supports adding adapters using either a dynamic or fixed id. The
latter is provided by aliases in the DT case. To prevent id collisions
of those two types, install this function which gives us the highest
fixed id, so we can then let the dynamically created ones come after
this highest number.
Signed-off-by: NWolfram Sang <wsa+renesas@sang-engineering.com>
Acked-by: NRob Herring <robh@kernel.org>
Signed-off-by: NWolfram Sang <wsa@the-dreams.de>
"

Add it also to U-Boot for DM I2C support.
Signed-off-by: NMichal Simek <michal.simek@xilinx.com>
Reviewed-by: NHeiko Schocher <hs@denx.de>
Reviewed-by: NSimon Glass <sjg@chromium.org>