remoteproc: k3_system_controller: Increase rx timeout

There is one case where 400ms is not sufficient for loading the
system firmware:
- System firmware is not signed with rsa degenerate key.
- ROM loading the sysfw directly from SPI flash which is in memory
  mapped mode.

The above scenario is definitely not desired in production use cases
as it effects boot time. But still keeping this support as this is
a valid boot scenario.

Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>