aic7xxx: make target mode enable a device hint

Previously it was only possible to enable target mode for these drivers
by rebuilding the kernel with AHC_TMODE_ENABLE or AHD_TMODE_ENABLE and a
bitmask of which units to statically enable for target mode.

There is no space-savings in the driver by not having AHC_TMODE_ENABLE
set, so in addition to the compile time option lets also introduce some
tunables:

hint.ahc.<unit>.tmode_enable=0/1
hint.ahd.<unit>.tmode_enable=0/1

For compatibility the old behavior is retained, but it can be overridden
with tunables

Signed-off-by: HP van Braam <hp@tmm.cx>
Reviewed by: imp, mav
Pull Request: https://github.com/freebsd/freebsd-src/pull/1208