powerpc/sgy_cts1000: convert to using gpiod API and facelift

This patch converts the driver to newer gpiod API, and away from
OF-specific legacy gpio API that we want to stop using.

While at it, let's address a few more issues:

- switch to using dev_info()/pr_info() and friends
- cancel work when unbinding the driver

Note that the original code handled halt GPIO polarity incorrectly:
in halt callback, when line polarity is "low" it would set trigger to
"1" and drive halt line high, which is counter to the annotation.
gpiod API will drive such line low. However I do not see any DTSes
in mainline that have a DT node with "sgy,gpio-halt" compatible.

Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/YzNNznewTyCJiGFz@google.com