power_domain: intel_adsp: code update

This patch contains several small changes to the intel adsp power
domain.

- include missing header,
- replacing sys_write32/sys_read32 with sys_write16/sys_read16 since
  DfPWRCTL is a 16 bit register,
- renaming struct to be more representing what it is,
- passing register address, not a value to the sys_read/sys_write
  functions,
- pd_intel_adsp_init is now returning actual status.

Signed-off-by: Tomasz Leman <tomasz.m.leman@intel.com>