ARM: dts: msm8974-FP2: add reboot-mode node

This enables userspace to signal the bootloader to go into the
bootloader or recovery mode.

The magic values can be found in both the downstream kernel and the LK
kernel (bootloader).

Reviewed-by: Brian Masney <masneyb@onstation.org>
Signed-off-by: Luca Weiss <luca@z3ntu.xyz>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>