rtc: select 24-hour mode at adoption, and refuse a 12-hour read #51

ZephCore never selects 24-hour mode on an I2C RTC, and it decodes a 12-hour PM hour as a wrong 24-hour one.

zephcore_rtc_restore() and zephcore_rtc_save() (zephcore/adapters/clock/ZephyrRTCDiscover.c) assume 24-hour mode, and the header says so: "12-hour mode is not decoded". Nothing sets or checks the mode. If other firmware or a tool has left the chip in 12-hour mode:

• Read. Hours are masked & 0x3F and must be BCD 00-23. In 12-hour mode bit 5 is AM/PM, and bits 4:0 hold 01-12. So PM 1-3 (21h-23h) decode as 21:00-23:00, eight hours ahead, and are accepted. PM 4-12 (24h-32h) fail the range check, so no time is taken. AM hours decode correctly.
• Write. The save writes 24-hour BCD, which a 12-hour chip reads as a different hour. 15:00 (15h), for example, is an invalid 12-hour value.

Which chips: the RV3028 (Control 2, 10h, bit 1 12_24) and the DS3231/DS3232 (hours register bit 6). Check the PCF8563 and RX8130CE data sheets for whether they have a 12-hour mode at all.

Parity. Zephyr's rtc_rv3028 writes Control 2 = 00h (24h) at init. MeshCore (meshcore-dev/MeshCore#3421, 6033e3e4) reads Status and Control 2 with the time, clears 12_24 when it is set and re-reads, and selects 24h before a write. ZephCore is the odd one out.

Proposed fix. Select 24-hour mode when a chip is adopted, for each chip that has the bit, and refuse a time read while the chip reports 12-hour mode. This is kept separate from liquidraver#103 (the RV3028 A0h year mark), which is upstream as agreed.