dt-bindings: nvmem: add binding for QTI SPMI SDAM

QTI SDAM allows PMIC peripherals to access the shared memory that is
available on QTI PMICs. Add documentation for it.

Signed-off-by: Shyam Kumar Thella <sthella@codeaurora.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Link: https://lore.kernel.org/r/20200116161100.30637-3-srinivas.kandagatla@linaro.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>