serial: sprd: keep console alive even if missing the 'enable' clock

serial: sprd: keep console alive even if missing the 'enable' clock

The sprd serial console can work with only 26M fixed clock,
but the probe() is returning fail if the clock "enable" is not
configured in device tree.

This patch will fix the problem to let the uart device which is
used for console can be initialized even missing "enable" clock
configured in devicetree. We should make sure the debug function
as available as we can.

Signed-off-by: Chunyan Zhang <chunyan.zhang@unisoc.com>
Signed-off-by: Chunyan Zhang <zhang.lyra@gmail.com>
Reviewed-by: Baolin Wang <baolin.wang@linaro.org>
Tested-by: Baolin Wang <baolin.wang@linaro.org>
Link: https://lore.kernel.org/r/20190826072929.7696-4-zhang.lyra@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>