mtd: onenand: samsung: drop Exynos4 and describe driver in KConfig

None of supported Samsung Exynos4 SoCs (Exynos4210, Exynos4412) seem to
use OneNAND driver so drop it.  Describe better which driver applies to
which SoC, to make configuring kernel for Samsung SoC easier.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20210924133223.111930-1-krzysztof.kozlowski@canonical.com