am335x, guardian: code cleanup and boot optimization

- remove redundant headers and boot modes from board file

Signed-off-by: Gireesh Hiremath <Gireesh.Hiremath@in.bosch.com>
Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>
Link: https://lore.kernel.org/r/20210611161350.2141-9-Gireesh.Hiremath@in.bosch.com