target-openrisc: Removes a non-conforming behavior for the first page of the memory

Where *software* leaves 0x0000 - 0x2000 unmapped, the hardware should
still allow for this area to be mapped.

Signed-off-by: Sebastian Macke <sebastian@macke.de>
Signed-off-by: Stefan Kristiansson <stefan.kristiansson@saunalahti.fi>
Reviewed-by: Jia Liu <proljc@gmail.com>