target/cortex_a: emit a clear error message when dbgbase can't be detected

In some cases (the most obvious are TI's SoCs) ROM table lacks entries
for the cores, so OpenOCD has no way to determine what debug base to
use. Due to an error fixed in ec9ccaa28849 it wasn't handled properly,
and OpenOCD would continue to try using dbgbase = 0, which happened to
work for e.g. AM437x.

This patch adds a clear indication to the user that to access such a
target, dbgbase must be set manually in the config.

Reported by Felipe Balbi on IRC.

Change-Id: Id8533e708f44b76550eb8b659564f5f45717c298
Signed-off-by: Paul Fertser <fercerpav@gmail.com>
Reviewed-on: http://openocd.zylin.com/2603
Tested-by: jenkins