board: ti: am57xx: Fix detection of board version

board_is*("rev", board_ti_get_rev()) uses strncmp() for
revison detection and assumes it is success if return value
is <= 0. This will fail in case of multiple versions, as
revb will be true for board_is_*revb() and board_is_*reva().
Fix it by looking for exact match of the string.

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