rtc: s35390a: clarify INT2 pin output modes

Fix the INT2 mode mask to not include the "TEST" flag. Furthermore
remove the not needed reversion of bits when parsing the INT2 modes.
Instead reverse the INT2_MODE defines to match the bit order from the
datasheet.

Additionally mention the flag names from the datasheet for the different
modes in the comments.

Signed-off-by: Richard Leitner <richard.leitner@skidata.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>