mfd: exynos-lpass: Add hardware dependency

This driver is meant for Exynos systems so do not offer the option
elsewhere unless build-testing.

Cc: Inha Song <ideal.song@samsung.com>
Cc: Beomho Seo <beomho.seo@samsung.com>
Signed-off-by: Jean Delvare <jdelvare@suse.de>
Reviewed-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>