Support the DECLARE_PHY_INTERFACE_MASK() macro that is used to declare
a bitmap by converting the macro to DECLARE_BITMAP(), as has been done
for the __ETHTOOL_DECLARE_LINK_MODE_MASK() macro.

This fixes a 'make htmldocs' warning:

include/linux/phylink.h:82: warning: Function parameter or member 'DECLARE_PHY_INTERFACE_MASK(supported_interfaces' not described in 'phylink_config'

that was introduced by commit
  38c310eb ("net: phylink: add MAC phy_interface_t bitmap")
Signed-off-by: NRandy Dunlap <rdunlap@infradead.org>
Reported-by: NStephen Rothwell <sfr@canb.auug.org.au>
Cc: Russell King (Oracle) <linux@armlinux.org.uk>
Link: https://lore.kernel.org/r/45934225-7942-4326-f883-a15378939db9@infradead.orgSigned-off-by: NJonathan Corbet <corbet@lwn.net>