dt-bindings: spmi: add SPMI PMIC arbiter debug bus bindings

Add binding documentation for the SPMI debug bus found on SPMI
PMIC arbiter version 5 and above.  This debug bus has read and
write access to all PMIC peripherals regardless of ownership
configurations.  It cannot be used on production devices because
it is disabled by an eFuse.

Change-Id: I344ddea23ecb09bede5cbdb19197ab52b24dda44