memory: samsung: describe drivers in KConfig

Rephrase the Kconfig option and make it clear it applies only to Samsung
SoC.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Link: https://lore.kernel.org/r/20210924133406.112174-1-krzysztof.kozlowski@canonical.com