reset: sti: Use devm_kcalloc() in syscfg_reset_controller_register()

* A multiplication for the size determination of a memory allocation
  indicated that an array data structure should be processed.
  Thus use the corresponding function "devm_kcalloc".

* Replace the specification of a data structure by a pointer dereference
  to make the corresponding size determination a bit safer according to
  the Linux coding style convention.

* Delete the local variable "size" which became unnecessary with
  this refactoring.

Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>