ARC: configs: cleanup

 - Remove CONFIG_DEFAULT_HOSTNAME from defconfigs

   There's no reason to set the same hostname to all ARC boards
   by default. It usually gets overwritten by init scripts anyways.

 - Remove disabled CONFIG_DEVKMEM from defconfigs

   It is disabled by default

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