iopoll: unify atomic and non-atomic interfaces

readl_poll_timeout and readl_poll_timeout_atomic really accomplish the
same thing, just in different contexts.  Unify their interfaces to
reduce cognitive load on developers and code reviewers.

Change-Id: I319db7cb3894c66447b3337c6802b723a38b3544
[veeras@codeaurora.org: As part of 3.18 upgrade,
remove all non-display-related code from this commit
	arch/arm/mach-msm/clock-mdss-8974.c
	drivers/iommu/arm-smmu.c
	include/linux/iopoll.h]
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>
Signed-off-by: Veera Sundaram Sankaran <veeras@codeaurora.org>