net: ethernet: update dependency and help text of mvneta

With the introduction of the support for Armada 375 and Armada 38x,
the hidden Kconfig option MACH_ARMADA_370_XP is being renamed to
MACH_MVEBU_V7. Therefore, the dependency that was used for the mvneta
driver can no longer work. This commit replaces this dependency by a
dependency on PLAT_ORION, which is used similarly for the mv643xx_eth
driver.

In addition to this, it takes this opportunity to adjust the
description and help text to indicate that the driver can is also used
for Armada 38x. Note that Armada 375 cannot use this driver as it has
a completely different networking unit, which will require a separate
driver.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: Jason Cooper <jason@lakedaemon.net>
Signed-off-by: David S. Miller <davem@davemloft.net>