platform/x86: Rename silead_dmi to touchscreen_dmi

Not only silead touchscreens need some extra info not available in the
ACPI tables to work properly. X86 devices with a Chipone ICN8505 chip also
need some DMI based extra configuration.

There is no reason to have separate dmi config code per touchscreen
controller vendor. This commit renames silead_dmi to a more generic
touchscreen_dmi name (and Kconfig option) in preparation of adding
info for tablets with an ICN8505 based touchscreen.

Note there are no functional changes all code changes are limited to
removing references to silead where these are no longer applicable.

Acked-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Acked-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>