dell-led: move driver to drivers/platform/x86/dell-wmi-led.c

The dell-led driver handles a specific WMI GUID present on some Dell
laptops and as such it belongs in the x86 platform driver subsystem.
Source code is moved along with the relevant Kconfig and Makefile
entries, with some minor modifications:

  - Kconfig option is renamed from CONFIG_LEDS_DELL_NETBOOKS to
    CONFIG_DELL_WMI_LED,

  - the X86 Kconfig dependency is removed as the whole
    drivers/platform/x86 menu depends on it, so there is no need to
    duplicate it,

  - the name of the module's source file is removed from the header
    comment to avoid the need to update it in the future.

Signed-off-by: Michał Kępień <kernel@kempniu.pl>
Tested-by: Alex Hung <alex.hung@canonical.com>
Reviewed-by: Pali Rohár <pali.rohar@gmail.com>
Acked-by: Pavel Machek <pavel@ucw.cz>
Acked-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Jacek Anaszewski <jacek.anaszewski@gmail.com>