i3c: mipi-i3c-hci: Refactor PIO register initialization

i3c: mipi-i3c-hci: Refactor PIO register initialization

Move the PIO register setup logic out of hci_pio_init() into a new
helper, __hci_pio_init().  This refactoring prepares for Runtime PM
support by allowing PIO registers to be reinitialized independently
after resume.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Reviewed-by: Frank Li <Frank.Li@nxp.com>
Link: https://patch.msgid.link/20260113072702.16268-13-adrian.hunter@intel.com
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>