mm/x86: Add wildcard '*' option as memmap=nn*align:name

In order to allow for requesting a memory region that can be used for
things like pstore on multiple machines where the memory is not the same,
add a new option to the memmap=nn$ kernel command line.

The memmap=nn$addr will reserve nn amount of memory at the physical
address addr. To use this, one must know the physical memory layout and
know where usable memory exists in the physical layout.

Add a '*' option that will assign memory by looking for a range that can
fit the given size and alignment. It will start at the high addresses, and
then work its way down.

The format is:  memmap=nn*align:name

Where it will find nn amount of memory at the given alignment of align.
The name field is to allow another subsystem to retrieve where the memory
was found. For example:

  memmap=12M*4096:oops ramoops.mem_name=oops

Where ramoops.mem_name will tell ramoops that memory was reserved for it
via the wildcard '*' option and it can find it by calling:

  if (memmap_named("oops", &start, &size)) {
	// start holds the start address and size holds the size given

Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>