m68k: Increase initial mapping to 8 or 16 MiB if possible

If the size of the first memory chunk is at least 8 or 16 MiB increase the
initial mapping to 8 resp. 16 MiB instead of 4 MiB.
This makes it possible to
  1. Map more memory in the first node without running out of space for the
     page tables,
  2. Boot kernels that don't fit in 4 MiB (e.g. multi_defconfig).

Signed-off-by: Andreas Schwab <schwab@linux-m68k.org>

  - Add support for 8 MiB,
  - Store initial mapping size in head.S for later reuse,
  - Add comment about large kernels.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>