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

drivers/mtd/nand/onenand/Kconfig

@@ -33,11 +33,12 @@ config MTD_ONENAND_OMAP2
 
 config MTD_ONENAND_SAMSUNG
 	tristate "OneNAND on Samsung SOC controller support"
-	depends on ARCH_S3C64XX || ARCH_S5PV210 || ARCH_EXYNOS4 || COMPILE_TEST
+	depends on ARCH_S3C64XX || ARCH_S5PV210 || COMPILE_TEST
 	help
-	  Support for a OneNAND flash device connected to an Samsung SOC.
-	  S3C64XX uses command mapping method.
-	  S5PC110/S5PC210 use generic OneNAND method.
+	  Support for a OneNAND flash device connected to Samsung S3C64XX
+	  (using command mapping method) and S5PC110/S5PC210 (using generic
+	  OneNAND method) SoCs.
+	  Choose Y here only if you build for such Samsung SoC.
 
 config MTD_ONENAND_OTP
 	bool "OneNAND OTP Support"