ARC: [plat-eznps] Use dedicated COMMAND_LINE_SIZE

The default 256 bytes sometimes is just not enough.
We usually provide earlycon=... and console=... and ip=...
All this and more may need more room.

Signed-off-by: Noam Camus <noamc@ezchip.com>
Acked-by: Vineet Gupta <vgupta@synopsys.com>