scripts/kernel-doc: parse __ETHTOOL_DECLARE_LINK_MODE_MASK

scripts/kernel-doc: parse __ETHTOOL_DECLARE_LINK_MODE_MASK

The __ETHTOOL_DECLARE_LINK_MODE_MASK macro is a variant of
DECLARE_BITMAP(), used by phylink.h. As we have already a
parser for DECLARE_BITMAP(), let's add one for this macro,
in order to avoid such warnings:

	./include/linux/phylink.h:54: warning: Function parameter or member '__ETHTOOL_DECLARE_LINK_MODE_MASK(advertising' not described in 'phylink_link_state'
	./include/linux/phylink.h:54: warning: Function parameter or member '__ETHTOOL_DECLARE_LINK_MODE_MASK(lp_advertising' not described in 'phylink_link_state'

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/d1d1dea67a28117c0b0c33271b139c4455fef287.1592895969.git.mchehab+huawei@kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>