drivers/misc: Add Aspeed P2A control driver

Fixup compiler warnings:
 - 108 warning: ISO C90 forbids mixed declarations and code
 - 264 warning: unused variable 'value'
 - 335 warning: unused variable 'res'

Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Patrick Venture <venture@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>