x86_64: pmap: removed support code for mixed-size mappings.

x86_64: pmap: removed support code for mixed-size mappings.

Mixed-size mappings (e.g. composing a mapping from 4k and 2M pages) don't work
on x86 unless the physical and virtual addresses have matching alignments (a
2M mapping needs both a 2M aligned virtual and physical address).  Thus we
don't support this case in the default library OS.

The kernel doesn't care though and you can roll your own system where you
allocate physical memory that matches the alignment so that the first 2M
aligned virtual address lands on a 2M aligned physical address.

Signed-off-by: Simon Gerber <simon.gerber@hp.com>