dts: arm: st: u3: fix flash erase block size and max erase time

- Change erase-block-size from 8192 to 4096 bytes to match
the 4 KB page size of STM32U3 flash.
- Update max-erase-time from 5 ms to 14 ms according to
datasheet specifications.
- These changes ensure correct flash erase behavior
and timing on STM32U3 devices.

Signed-off-by: Khaoula Bidani <khaoula.bidani-ext@st.com>