ti: common: board_detect: Rename EEPROM scratch start macro

Non OMAP platforms i.e. Keystone will also need to use the board
EEPROM helpers so let's make the macro platform independent.

Signed-off-by: Roger Quadros <rogerq@ti.com>
Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>