watchdog: sp5100_tco: Use request_muxed_region where possible

Use request_muxed_region for multiplexed IO memory regions.
Also, SP5100_IO_PM_INDEX_REG/SP5100_IO_PM_DATA_REG are only
used during initialization; it is unnecessary to keep the
address range reserved.

Cc: Zoltán Böszörményi <zboszor@pr.hu>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>