dts: atmel sam4s: fix sram address

Changed Atmel SAM4S series sram adress from 0x20100000 to 0x20000000. Now
it matches what is in Atmel SAM4S Series Datasheet chapter 8
section 1.1 Internal SRAM:

https://ww1.microchip.com/downloads/aemDocuments/documents/OTH/ProductDocuments/DataSheets/Atmel-11100-32-bitCortex-M4-Microcontroller-SAM4S_Datasheet.pdf#G1.1069257



Fixes #85211.

Signed-off-by: Michal Myczkowski <mmyczkowski@antmicro.com>
(cherry picked from commit 3a53845f)