ARC: [plat-axs103] Enable loop block devices

ARC: [plat-axs103] Enable loop block devices

As mentioned in LTP's README.ARC:
------------->8------------
 Requirements for the environment

* Linux must be built with support of loop block devices. Thus it's
  necessary to enable these Linux kernel options:
  CONFIG_BLK_DEV
  CONFIG_BLK_DEV_LOOP
------------->8------------
enabling loop block devices.

That among other things lead to additional 10 fatal signals
appearing during LTP run.

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>