gpio: samsung: move gpio-samsung driver back to platform code

gpio: samsung: move gpio-samsung driver back to platform code

The gpio-samsung driver is special in the sense that it
interacts directly in multiple ways with the legacy platform
code for the s3c24xx and s3c64xx platforms. In contrast,
all devicetree based machines for Samsung, including the
ones on those two SoC families use a different driver.

The header files that define the interface between the platform
code and the gpio driver are not visible when building a
kernel for ARCH_MULTIPLATFORM, which prevents us from
turning on this option for s3c64xx.

To work around this, we now move the driver back into platform
code, from where it was originally moved to as part of commit
1b39d5f2cc5c28 ("gpio/samsung: gpio-samsung.c to support
Samsung GPIOs").

The long-term plan for this driver would be to remove it
entirely, after all Samsung machines have been converted
over to boot from DT, but there is currently no timeline
for when that might happen.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>