boards: esp32: M5Stack Core2 has PCF8563 RTC, not PCF8523

The RTC chip on i2c0 is a PCF8563, not a PCF8523.
RTC _almost_ works when using the latter, but not quite, hence why it
probably was missed before.
Fix tested as working using RTC Shell commands.

Signed-off-by: Benjamin Cabé <benjamin@zephyrproject.org>