MFC	r259007

Increase PHYS_AVAIL_SZ because on pSeries machines we can have many logical
regions which represent the total amount of memory. The size of these regions
is not the physical size of the chip but it is a logical one and it is given
by the OpenFirmware, it is selectable at boot time and varies between 16MB and
256MB in my case. There is an 'automatic' option which would select the size as
64MB in case you have around 16GB of RAM.
To make sure we can allocate RAM with the automatic option bump this value
of PHYS_AVAIL_SZ to 256.