net: dsa: mv88e6xxx: Introduce _mv88e6xxx_phy_page_{read, write}

Add versions of the phy_page_read and _write functions to
be used in a context where the SMI mutex is held.

Tested-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Reviewed-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: Patrick Uiterwijk <patrick@puiterwijk.org>
Signed-off-by: David S. Miller <davem@davemloft.net>