fix(stm32mp1): update the FIP load address for serial boot

fix(stm32mp1): update the FIP load address for serial boot

Update the FIP load address and size for serial boot to support
product with a DDR size = 128MB
1/ Move the FIP location at the end of the first 128MB
2/ Reduce the DWL_BUFFER_SIZE to 16MB, to be coherent with the value
   indicated in USB enumeration
   - for STM32MP13x: "@SSBL /0x03/1*16Me"
   - for STM32MP15x: "@Partition3 /0x03/1*16Me"

Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Change-Id: Id93bf00c64832c17426bfd78e060861275677ecc