memory commands: separate into one file per command ...

Normally in commands we have one file per command which is named like
the command itself. The memory commands are an exception to this. This
patch changes this by separating the memory commands. This also has the
effect that the memory commands can now be selected individually. Along
the way we add some Kconfig help text for the commmands.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>