configs: am335x_guardian: disable spl command

- disable support for spl command

Signed-off-by: Gireesh Hiremath <Gireesh.Hiremath@in.bosch.com>
Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>
Link: https://lore.kernel.org/r/20210611161350.2141-11-Gireesh.Hiremath@in.bosch.com