ti: common: board_detect: Add function to determine if EEPROM was read

When the EEPROM is first read its contents are stored in memory as a
cache to avoid further I2C operations. To determine if the EEPROM was
previously read the easiest way is to check the memory to see if the
EEPROM's magic header value is set. Create a new function that can
determine if the EEPROM was previously read or not without having to
perform a I2C transaction.

Signed-off-by: Franklin S Cooper Jr <fcooper@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>