eeprom: merge cmdline parsing of eeprom commands

Merge the parsing of layout aware and layout unaware eeprom commands into
one parsing function. With this change, layout aware commands now follow
the eeprom read and eeprom write conventions of making i2c bus and i2c address
parameters optional.

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>