ARM: dts: msm: add SPMI debug bus for sm8150 platforms

Add an SPMI debug bus device and associated child devices so that
consumers can have unrestricted access to the PMICs on
pre-production devices.  This helps to make debugging easier.

Change-Id: I1cbcc8756afb878d07e57fc86ced93748eed7636
Signed-off-by: Subbaraman Narayanamurthy <subbaram@codeaurora.org>