ti/common: add support for extension_scan_board function

The BeagleBone platforms all use a common mechanism to discover and
identify extension boards (called "capes"): each extension board has an
I2C-connected EEPROM describing itself.

This patch implements a generic extension_scan_board() feature that can
be used by all BeagleBone platforms to read those I2C EEPROMs and fill
in the list of "extension" structures.

Following commits will enable this common logic on two BeagleBone
platforms.

Signed-off-by: Kory Maincent <kory.maincent@bootlin.com>