configs: am335x_guardian: add ubi fastmap support

- Trigger fastmap automatically

Signed-off-by: Gireesh Hiremath <Gireesh.Hiremath@in.bosch.com>
Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>
Link: https://lore.kernel.org/r/20210611161350.2141-4-Gireesh.Hiremath@in.bosch.com