spi: s3c64xx: remove s5p64x0 related spi codes

This patch removes s5p64x0 related spi because of no more support for
s5p64x0 SoCs. Meanwhile, cleanup SPI DT bindings for s5p6440-spi, it
should be s5p64x0-spi instead.

Cc: Mark Brown <broonie@linaro.org>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>