msm: board-8064: Add control for PM8921 regulators S8, L21, and L29

Add rpm-regulator configurations so that PM8921 regulators S8,
L21, and L29 may be controlled by Linux consumers on APQ 8064
targets.

These three regulators are available when using discrete PMICs
PM8921 + PM8821, but not when using the PMM8920 PMIC module.  On
the PMM8920, these regulators are not pinned out.

Change-Id: Iebf8ea372a55a14860ef10e895c31d5ae0a43d1b
Signed-off-by: David Collins <collinsd@codeaurora.org>