Defaulting to DEBUG_ICEDCC will cause systems to hang during boot unless
a hardware debugger is listening to the debug comms. channel.

This patch adds a dummy UART option as the default DEBUG_LL choice which
requires the platform to do the right thing.
Acked-by: NStephen Boyd <sboyd@codeaurora.org>
Tested-by: NMarc Zyngier <marc.zyngier@arm.com>
Signed-off-by: NWill Deacon <will.deacon@arm.com>
Signed-off-by: NRussell King <rmk+kernel@arm.linux.org.uk>