ARM: dts: qcom: msm8974-FP2: Add notification LED

FP2 has a RGB LED connected to the TRILED and hence channels 7, 6 and
5 of the LPG. Add a node describing this.

Signed-off-by: Luca Weiss <luca@z3ntu.xyz>
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Link: https://lore.kernel.org/r/20220505163029.6541-1-luca@z3ntu.xyz