arm/airoha: partly revert support of multiple RAM size for an7581

arm/airoha: partly revert support of multiple RAM size for an7581

Partly revert commit 726404a66c773 ("airoha: rework RAM size handling
to support multiple RAM size").

The general idea is good, but the call of

  get_ram_size((void *)gd->ram_base, SZ_8G);

produces the following result on my an7581 board with only 1Gb of
memory:

  "Synchronous Abort" handler, esr 0x96000004
  elr: 0000000081e201c8 lr : 0000000081e20160 (reloc)
  elr: 00000000bff501c8 lr : 00000000bff50160
  x0 : 0000000180000000 x1 : 0000000100000000
  x2 : 000000000000002e x3 : 0000000000000002
  x4 : 000000001fbf0000 x5 : 0060000000000401
  x6 : 0000000000000000 x7 : 00000000bffdb268
  x8 : 0000000000000060 x9 : 00000000bffdb2c8
  x10: 0000000000000000 x11: 0000000000000060
  x12: 00000000bffdb268 x13: 00000000841c56d0
  x14: 00000000841c56d0 x15: 0000000000000000
  x16: 00000000841506e4 x17: dd7fe29aec3b07e8
  x18: 00000000bf710e00 x19: 0000000080000000
  x20: 0000000000000000 x21: 0000000020000000
  x22: 0000000200000000 x23: 0000000000000001
  x24: 0000000040000000 x25: 00000000bf708e78
  x26: 00000000bf7bdca0 x27: 0000000000000000
  x28: 0000000000000000 x29: 00000000bf708e20

  Code: 910943ff d65f03c0 d37df2a1 8b150e60 (f8616a62)
  Resetting CPU ...

So just not call get_ram_size() and use the value from dtb.

Fixes: 726404a66c773 ("airoha: rework RAM size handling to support multiple RAM size")
Signed-off-by: Mikhail Kshevetskiy <mikhail.kshevetskiy@iopsys.eu>