Make the CONFIG symbols tristate and add help text.

The broadcom and Microchip KSZ tag drivers support two different
tagging protocols in one driver. Add a configuration option for the
drivers, and then options to select the protocol.

Create a submenu for the tagging drivers.
Signed-off-by: Andrew Lunn <andrew@lunn.ch>

v2:
tab/space cleanup
Help text wording
NET_DSA_TAG_BRCM_COMMON and NET_DSA_TAG_KZS_COMMON hidden

v3:
More tabification
Punctuation

v4:
trailler->trailer
Signed-off-by: David S. Miller <davem@davemloft.net>