phy: core: Document function args

phy: core: Document function args

Some function arguments are missing from documentation prompting
validation kernel doc script to complain:

drivers/phy/phy-core.c:1078: warning: Function parameter or member
'children' not described in '__devm_of_phy_provider_register'
drivers/phy/phy-core.c:1125: warning: Function parameter or member
'phy_provider' not described in 'devm_of_phy_provider_unregister'

Add the documentation for these

Link: https://lore.kernel.org/r/20200629145010.122675-2-vkoul@kernel.org
Signed-off-by: Vinod Koul <vkoul@kernel.org>