Implement the read() function in the early console driver. With
recently added kgdboc_earlycon feature, this allows you to use kgdb
to debug fairly early into the system boot.

We only bother implementing this if polling is enabled since kgdb can't
be enabled without that.
Signed-off-by: Sumit Garg <sumit.garg@linaro.org>
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Link: https://lore.kernel.org/r/20200507130644.v4.12.I8ee0811f0e0816dd8bfe7f2f5540b3dba074fae8@changeidSigned-off-by: Daniel Thompson <daniel.thompson@linaro.org>