phytool: add phytool utility

phytool: add phytool utility

From the README.md:

phytool is a Linux MDIO register access utility.

Usage:

phytool read  IFACE/ADDR/REG
phytool write IFACE/ADDR/REG <0-0xffff>
phytool print IFACE/ADDR[/REG]

where

ADDR := C22 | C45
C22  := <0-0x1f>
C45  := <0-0x1f>:<0-0x1f>
REG  := <0-0x1f>

Note: Not all MDIO drivers support the port:device
      Clause 45 address format.

The read and write commands are simple register level accessors.
The print command will pretty-print a register. When using the
print command, the register is optional. If left out, the most
common registers will be shown.

Signed-off-by: Christian Lamparter <chunkeey@gmail.com>