board: am57xx: Fix missing check for beagle_x15

When beagleboard-X15 is booted, we see the following log:
Unidentified board claims BBRDX15_ in eeprom header

This is because of the missing check for x15 (the default) and reports
an error for a valid board configuration. Fix the same.

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