Fix the RTC device model to operate correctly in 12-hour mode. The following
table documents the values in the RTC 'hour' field in the two modes:

Hour-of-the-day 12-hour mode 24-hour mode
12 AM 12 0
[1-11] AM [1-11] [1-11]
12 PM 0x80 | 12 12
[1-11] PM 0x80 | [1-11] [13-23]

Reported by: Julian Hsiao (madoka@nyanisore.net)
MFC after: 1 week