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.

CRs-Fixed: 834681
Change-Id: I0a3f80afb5b91305bcd9ad1271d516bd3f79bcba
Signed-off-by: Mao Jinlong <c_jmao@codeaurora.org>