mtd: denali_pci: refactor driver using devres API

In recent kernels we have a lot of helper functions, including
devres API, to make life of device driver developer easy.

Convert the driver using devm_kzalloc() and pcim_enable_device().

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>