watchdog: sp5100_tco: Always use SP5100_IO_PM_{INDEX_REG,DATA_REG}

SP5100_IO_PM_INDEX_REG and SB800_IO_PM_INDEX_REG are used inconsistently
and define the same value. Just use SP5100_IO_PM_INDEX_REG throughout.
Do the same for SP5100_IO_PM_DATA_REG and SB800_IO_PM_DATA_REG.
Use helper functions to access the indexed registers.

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