board: ti: beagleboneai: emmc read changes

BeagleBoard.org BeagleBone AI rev A1 does not include a board
identifier I2C EEPROM due to a design oversight. These boards have
been put into production and are generally available now.

The board identifier information, however, has been included in the
second eMMC linear boot partition (/dev/mmcblk1boot1).

This patch works by:
* First, looking for a board identifier I2C EEPROM and if not found,
* Then seeing if the boot mode matches BeagleBone AI with eMMC in the
  boot chain to make sure we don't enable eMMC pinmuxes on boards
  that don't support it, and
* Finally, initializes the eMMC pins and reading the header.

Signed-off-by: Jason Kridner <jdk@ti.com>
Signed-off-by: Caleb Robey <c-robey@ti.com>
Cc: Robert Nelson <robertcnelson@gmail.com>
Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>