dt-bindings: spmi: Correct 'reg' schema

'reg' is defined to be N address entries of M cells each. For SPMI, N is 1
and M is 1 or 2. The schema fails to define the number of entries as it
only specifies the inner cell(s). To fix, add an outer items list with 1
entry.

Cc: Stephen Boyd <sboyd@kernel.org>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20210615172024.856360-1-robh@kernel.org