kernel: mm: rename Z_VIRT_RAM_* to K_MEM_VIRT_*

Renames:
  Z_VIRT_RAM_START to K_MEM_VIRT_RAM_START
  Z_VIRT_RAM_SIZE to K_MEM_VIRT_RAM_SIZE
  Z_VIRT_RAM_END to K_MEM_VIRT_RAM_END

This is part of a series to move memory management related
stuff from Z_ namespace into its own namespace.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>