powerpc/fsl-booke-64: Don't limit ppc64_rma_size to one TLB entry

This is required for kdump to work when loaded at at an address that
does not fall within the first TLB entry -- which can easily happen
because while the lower limit is enforced via reserved memory, which
doesn't affect how much is mapped, the upper limit is enforced via a
different mechanism that does.  Thus, more TLB entries are needed than
would normally be used, as the total memory to be mapped might not be a
power of two.

Signed-off-by: Scott Wood <scottwood@freescale.com>