tcl: add bcm47xx config and Asus RT-N16 board using it

tcl: add bcm47xx config and Asus RT-N16 board using it

This adds the bcm47xx config with the special undocumented trick to
put it into standard EJTAG mode from the mystic "LV mode".

The RAM setup is not done as it would require considerable efforts
without much practical gain.

The only issue I noticed so far is that "reset" doesn't actually reset
the chip.

Unfortunately, it's unclear how to make it work properly with SRST as
OpenOCD asserts it in MIPS-specific code so the device will enter LV
mode again but the LV tap is already disabled by that time, so it's
not possible to send the magic command again.

Anyway, this config is more than enough to "recover" any RT-N16
provided the hardware is not damaged.

Change-Id: I0894e339763e6d20d1c93341c597382b479d039b
Signed-off-by: Paul Fertser <fercerpav@gmail.com>
Reviewed-on: http://openocd.zylin.com/1849
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>