chmem: add chmem documentation for dynamic (de)configuration of memory

chmem: add chmem documentation for dynamic (de)configuration of memory

Describe chmem configure, deconfigure and memmap-on-memory options:

ff18dcb19aab ("s390/sclp: Add support for dynamic (de)configuration of
memory") s390 kernel no longer pre-adds all standby memory at boot.
Instead, users must explicitly configure a block before it can be used
for online/offline actions. At configuration time, users can dynamically
decide whether to use optional memmap-on-memory for each memory block,
where value of 1 allocates metadata (such as struct pages array) from
the hotplug memory itself, enabling hot-add operations even under memory
pressure. A value of 0 stores metadata in regular system memory, which
may require additional free memory, but enables continuous physical
memory across memory blocks.

Add documentation to reflect the following options:
* chmem --configure 128M --memmap-on-memory 1
* chmem --deconfigure 128M
* chmem --enable 128M # implicitly configure memory if supported by
  architecture and online it
* chmem --disable 128M  # offline memory and implicitly deconfigure if
  supported by the architecture.

Just like online and offline actions, memory configuration and
deconfiguration can be controlled through similar options. Also,
memmap-on-memory setting can be changed, only when the memory block is
in deconfigured state. This means, it is usable only via --configure
option.

Reviewed-by: Maria Eisenhaendler <maria1@de.ibm.com>
Signed-off-by: Sumanth Korikkar <sumanthk@linux.ibm.com>