mii: add read-modify-write option to mii command

When accessing PHY registers it is often desirable to only update
selected bits, so it is necessary to first read the current value
before writing back an modified value with the relevant bits
updated.

To simplify this and to allow such operations to be incorporated
into simple shell scripts propose adding a 'modify' option to the
existing mii command, which takes a mask indicating the bits to
be updated in addition to a data value containing the new bits,
ie, <updated> = (<data> & <mask>) | (<current> & ~<mask>).

Signed-off-by: Tim <tim.james@macltd.com>
Cc: Nobuhiro Iwamatsu <iwamatsu@nigauri.org>
Cc: Joe Hershberger <joe.hershberger@gmail.com>
Cc: Jeroen Hofstee <jeroen@myspectrum.nl>
Cc: Tom Rini <trini@konsulko.com>
Cc: Tim <tim.james@macltd.com>