Add memory modify command (mm) ...

As of now we have no way to manipulate individual bits of registers on the
command line. This introduces a memory modify command which allows this. It
has the syntax:

mm [OPTIONS] <adr> <val> <mask>

With [OPTIONS] being the usual memory command options (-b, -w, -l, -d <file>).

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