gpio: sifive: add support for DM based gpio driver for FU540-SoC

gpio: sifive: add support for DM based gpio driver for FU540-SoC

This patch adds a DM based driver model for gpio controller present in
FU540-C000 SoC on HiFive Unleashed A00 board. This SoC has one GPIO
bank and 16 GPIO lines in total, out of which GPIO0 to GPIO9 and
GPIO15 are routed to the J1 header on the board.

This implementation is ported from linux based gpio driver submitted
for review by Wesley W. Terpstra <wesley@sifive.com> and/or Atish Patra
<atish.patra@wdc.com> (many thanks !!). The linux driver can be referred
here [1]

[1]: https://lkml.org/lkml/2018/10/9/1103

Signed-off-by: Sagar Shrikant Kadam <sagar.kadam@sifive.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>