input: qpnp-power-on: set boot_reason when system-reset exists

Some platforms don't configure qcom,secondary-pon-reset like 8998.
So set boot_reason when "qcom,system-reset" node exists to avoid
setting a wrong value to boot_reason.

CRs-Fixed: 1102732
Change-Id: I9e9ff2f2d0ffac6baa5d0663664001eb30638e87
Signed-off-by: Mao Jinlong <c_jmao@codeaurora.org>