ARC: HSDK: Enable SPI flash support

HSDK board has sst26wf016 SPI flash IC which we want to support.

Add SPI controller, CS-gpio and SPI flash nodes to hsdk device tree.
Enable corresponding options in hsdk defconfig.

For SPI write functionality to work we need [1] which
adds support of sst26xxx ICs.

[1] https://patchwork.ozlabs.org/project/uboot/list/?series=35796



Signed-off-by: Eugeniy Paltsev <Eugeniy.Paltsev@synopsys.com>
Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>