powerpc: respect mem= setting for early memory limit setup

For those MMUs that have some form of bolt'd linear mapping (TLB)
required its rare that one ever sets mem= smaller than the size of that
mapping.

However, on Book-E 64 parts the initial linear mapping is quite large
(1G) so its quite reasonable that mem= is set smaller than that.

We need to parse the command line for mem= limit and constrain the
amount of memory we map initially by it if need be.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>