platform/x86: dell-wmi: add new backlight events

Add events with a type of 0x0010 and a code of 0x57 / 0x58,
this silences the following messages being logged on a
Dell Inspiron 5593:

dell_wmi: Unknown key with type 0x0010 and code 0x0057 pressed
dell_wmi: Unknown key with type 0x0010 and code 0x0058 pressed

These are brightness events and will be handled by acpi-video

Signed-off-by: Y Paritcher <y.linux@paritcher.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>