power: qpnp-smbcharger: add a module parameter for parallel charging

Add a module parameter "parallel_en" to the qpnp-smbcharger driver for
enabling and disabling the parallel charging feature.

This will allow greater flexibility for testing the parallel charging
feature.

Change-Id: I43f33195bf5d51a83d086f07dc4f064060cadd43
Signed-off-by: Xiaozhe Shi <xiaozhes@codeaurora.org>