msm: qpnp-power-on : avoid overriding value of boot_reason

Currently boot_reason is overridden since the secondary PMIC PON devices
are specified and probed. Update the boot_reason only after reading
primary PMIC's PON reason status.

Change-Id: I02d97e9b2c2a88c36671359e282b6870fd936a71
Signed-off-by: Mao Jinlong <c_jmao@codeaurora.org>