Documentation: ACPI: DSD: include phy.rst in the toctree

Include the new phy.rst into the index of the ACPI support
documentation.

Fixes: e71305ac ("Documentation: ACPI: DSD: Document MDIO PHY")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Ioana Ciornei <ioana.ciornei@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>