ARM: dts: imx51-apf51dev: Fix 'backlight@bl1' node with unit name and no reg property

The following build warning is seen with W=1:

Warning (unit_address_vs_reg): Node /backlight@bl1 has a unit name, but no reg property

Fix this warning by removing '@bl1'from such node and change 'bl1grp' to
'backlightgrp', once there is only one backlight in this dts.

Signed-off-by: Marco Franchi <marco.franchi@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>