mmc: sdhci-msm: configure regulators only if defined in dts

This patch adds support to configure regulators only if
it is defined in dts files. However, it doesn't return an
error otherwise.

Change-Id: Iac2284b2df3b3d8af623da0f17697694994b34b8
Signed-off-by: Asutosh Das <asutoshd@codeaurora.org>
[xiaonian@codeaurora.org: fix trivial merge conflict]
Signed-off-by: Xiaonian Wang <xiaonian@codeaurora.org>