arc: em_starterkit default changed to EM7D

ZEP-1317 records a problem were em_starterkit appears to boot up
EM7d, sometimes, due to a misread of the dip-switches.
This could be a mechanical issue. I am seeing it on my board too,
and I often have to struggle with the dip-switch to get it to boot
EM9D. For automated-testing, perhaps it is better if the default is
EM7D, since this is the all-switches-up and contact in the switch
isn't needed. Meanwhile, I'll work with our board team to try to get
to the bottom of this.

ZEP-1317

Change-Id: I41095c1d8b07ce156cc3856a50196c064af2ee18
Signed-off-by: Chuck Jordan <cjordan@synopsys.com>