board: siemens: iot2050: Pass DDR size from FSBL

Due to new DDR size introduction, the current logic of determining the
DDR size is not able to get the correct size.

Instead, the DDR size is determined by the FSBL(SEBOOT) then passed to
u-boot through the scratchpad info.

The SEBoot version must be >= D/V01.04.01.02 to support this change.

Also now for some variants, the DDR size may > 2GB, so borrow some code
from the TI evm to iot2050 to support more than 2GB DDR.

Signed-off-by: Baocheng Su <baocheng.su@siemens.com>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>

doc/board/siemens/iot2050.rst

@@ -29,6 +29,9 @@ The following binaries from that source need to be present in the build folder:
  - seboot_pg1.bin
  - seboot_pg2.bin
 
+Note that SE-Boot D/V01.04.01.02 or greater is required, otherwise the DDR size
+will not be picked up correctly by U-Boot.
+
 When using the watchdog, a related firmware for the R5 core(s) is needed, e.g.
 https://github.com/siemens/k3-rti-wdt. The name and location of the image is
 configured via CONFIG_WDT_K3_RTI_FW_FILE.