net: dsa: mv88e6xxx: NET_DSA_MV88E6XXX_PTP should depend on NET_DSA_MV88E6XXX

Making global2 support mandatory removed the Kconfig symbol
NET_DSA_MV88E6XXX_GLOBAL2.  This symbol also served as an intermediate
symbol to make NET_DSA_MV88E6XXX_PTP depend on NET_DSA_MV88E6XXX.  With
the symbol removed, the user is always asked about PTP support for
Marvell 88E6xxx switches, even if the latter support is not enabled.

Fix this by reinstating the dependency.

Fixes: 63368a74 ("net: dsa: mv88e6xxx: Make global2 support mandatory")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>