platform/x86: Move all dell drivers to their own subdirectory

A user without a Dell system doesn't need to pick any of these
drivers.

Users with a Dell system can enable this submenu and all drivers
behind it will be enabled.

Suggested-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Mario Limonciello <mario.limonciello@dell.com>
Link: https://lore.kernel.org/r/20210203195832.2950605-1-mario.limonciello@dell.com
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>