soc: aspeed: lpc-ctrl: make parameter optional

Making memory-region and flash as optional parameter in device
tree if user needs to use these parameter through ioctl then
need to define in devicetree.

Signed-off-by: Vijay Khemka <vijaykhemka@fb.com>
Reviewed-by: Andrew Jeffery <andrew@aj.id.au>
Signed-off-by: Joel Stanley <joel@jms.id.au>