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>

dts/arm/st/u3/stm32u3.dtsi

@@ -99,9 +99,9 @@
 				compatible = "st,stm32-nv-flash", "soc-nv-flash";
 
 				write-block-size = <16>;
-				erase-block-size = <8192>;
-				/* maximum erase time(ms) for a 8K sector */
-				max-erase-time = <5>;
+				erase-block-size = <4096>;
+				/* maximum erase time(ms) for a 4KB page */
+				max-erase-time = <14>;
 			};
 		};