jtag: drivers: bcm2835gpio: set 4ma drive, slow slew rate

Both the drive strength and slew rate are currently set to very high
values.  This causes the waveforms to overshoot and be less reliable
when operating at high speed.

This patch lowers the slew rate and sets the drive strength to 4ma,
improving the waveform output.

Change-Id: I761d35cd64bc54a9e94043904ef00a003b056af0
Signed-off-by: Sean Cross <sean@xobs.io>
Reviewed-on: http://openocd.zylin.com/3707


Tested-by: jenkins
Reviewed-by: Paul Fertser <fercerpav@gmail.com>
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-by: Philipp Guehring <pg@futureware.at>