Introduce parse_i2c_bus_addr() to generalize the parsing of i2c bus number and
i2c device address. This is done in preparation for merging layout aware and
layout unaware command parsing into one function.

No functional changes.

Cc: Heiko Schocher <hs@denx.de>
Cc: Marek Vasut <marex@denx.de>
Cc: Simon Glass <sjg@chromium.org>
Cc: Igor Grinberg <grinberg@compulab.co.il>
Cc: Tom Rini <trini@konsulko.com>
Signed-off-by: Nikita Kiryanov <nikita@compulab.co.il>