dm: modify vE820 to adapt to pSRAM for post-launched RTVM
When pSRAM is enabled for post-launched RTVM, we add a segment in vE820 for
pSRAM, and therefore the lowmem RAM will be split into part1 and part2.
Also, code of post vE820 initialization is refined
Tracked-On: #5330
Signed-off-by: Qian Wang <qian1.wang@intel.com>