Documentation: leds: Add MT6370 doc to the toctree

Documentation: leds: Add MT6370 doc to the toctree

Commit 4ba9df04b7ac66 ("docs: leds: Add MT6370 RGB LED pattern document")
adds documentation for Mediatek MT6370 RGB LED device, but forgets to
include it to leds toctree index.

Add the missing entry.

Link: https://lore.kernel.org/oe-kbuild-all/202303182310.tB1mUzU7-lkp@intel.com/
Fixes: 4ba9df04b7ac66 ("docs: leds: Add MT6370 RGB LED pattern document")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Bagas Sanjaya <bagasdotme@gmail.com>
Acked-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Lee Jones <lee@kernel.org>
Link: https://lore.kernel.org/r/20230319074903.13075-2-bagasdotme@gmail.com