drivers: qcom: cmd-db: modularize commandDB driver

Add support for compiling commandDB as module. Since the command DB data
is written to DDR by a remote processor and exists while the system is
active, removing the module does not make sense. Initialize the driver
as builtin platform driver.

Change-Id: I3e8909f556815b6831947661531f1e4d3c464a86
Signed-off-by: Lina Iyer <ilina@codeaurora.org>