MFC r270862, r270878: MMU fixes for kernel startup.

  Fix the handling of MMU type in the AP entry code.  The ARM_MMU_V6/V7
  symbols are always #defined to 0 or 1, so use #if SYM not #if defined(SYM).
  Also, it helps if you include the header file that defines the symbols.

  The Marvell PJ4B cpu family is armv7, not armv6.

Approved by:	re(gjb)