configs: am65x_evm_r5: Enable GPT support

The second loader stages may be stored on GPT partitions,
enable support for this here.

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