dsa: Allow tag drivers to be built as modules

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>