powerpc/prom: Remove limit on maximum size of properties

On a 16TB system (using AMS/CMO), I get:

WARNING: ignoring large property [/ibm,dynamic-reconfiguration-memory] ibm,dynamic-memory length 0x000000000017ffec

and significantly less memory is thus shown to the partition. As far as
I can tell, the constant used is arbitrary. Ben Herrenschmidt provided
additional background that

> The limit was originally set because of Apple machines carrying ROM
> images in the device-tree, at a time where we were much more memory
> constrained than we are now.

and that it is likely not very useful any longer.

Signed-off-by: Nishanth Aravamudan <nacc@us.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>