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>
diff --git a/board/ti/common/Kconfig b/board/ti/common/Kconfig
index 9ead7ca..49edd98 100644
--- a/board/ti/common/Kconfig
+++ b/board/ti/common/Kconfig
@@ -16,6 +16,12 @@
 	default 0x50
 	depends on TI_I2C_BOARD_DETECT
 
+config CAPE_EEPROM_BUS_NUM
+	int "Cape EEPROM's I2C bus address"
+	range 0 8
+	default 2
+	depends on CMD_EXTENSION
+
 config TI_COMMON_CMD_OPTIONS
 	bool "Enable cmd options on TI platforms"
 	imply CMD_ASKENV