ACPI / memhotplug: add parameter to disable memory hotplug

When booting a kexec/kdump kernel on a system that has specific memory
hotplug regions the boot will fail with warnings like:

 swapper/0: page allocation failure: order:9, mode:0x84d0
 CPU: 0 PID: 1 Comm: swapper/0 Not tainted 3.10.0-65.el7.x86_64 #1
 Hardware name: QCI QSSC-S4R/QSSC-S4R, BIOS QSSC-S4R.QCI.01.00.S013.032920111005 03/29/2011
  0000000000000000 ffff8800341bd8c8 ffffffff815bcc67 ffff8800341bd950
  ffffffff8113b1a0 ffff880036339b00 0000000000000009 00000000000084d0
  ffff8800341bd950 ffffffff815b87ee 0000000000000000 0000000000000200
 Call Trace:
  [<ffffffff815bcc67>] dump_stack+0x19/0x1b
  [<ffffffff8113b1a0>] warn_alloc_failed+0xf0/0x160
  [<ffffffff815b87ee>] ?  __alloc_pages_direct_compact+0xac/0x196
  [<ffffffff8113f14f>] __alloc_pages_nodemask+0x7ff/0xa00
  [<ffffffff815b417c>] vmemmap_alloc_block+0x62/0xba
  [<ffffffff815b41e9>] vmemmap_alloc_block_buf+0x15/0x3b
  [<ffffffff815b1ff6>] vmemmap_populate+0xb4/0x21b
  [<ffffffff8...