mpfs/opensbi: Add the ddr memory region back to opensbi ld script
fixes ld-envm-opensbi.script:41: warning: memory region `ddr' not declared
MEMORY
{
+ ddr (rx) : ORIGIN = 0x80000000, LENGTH = 4M /* w/ cache */
envm (rx) : ORIGIN = 0x20220100, LENGTH = 128K - 256 /* 256 reserved for hss headers */
l2lim (rwx) : ORIGIN = 0x08000000, LENGTH = 1024k
l2zerodevice (rwx) : ORIGIN = 0x0A000000, LENGTH = 512k
