powerpc: mpc85xx: Fix CONFIG_OF_SEPARATE support

powerpc: mpc85xx: Fix CONFIG_OF_SEPARATE support

Currently CONFIG_OF_SEPARATE is completely broken and U-Boot for some
mpc85xx board (e.g. P2020) has to be compiled with CONFIG_OF_EMBED.
Otherwise it crashes during early init.

When debug console is enabled and all debug logging options are turned on
then U-Boot on P2020 with CONFIG_OF_SEPARATE prints following error:

  No valid device tree binary found at 110dc300
  initcall sequence 110d3560 failed at call 1109535c (err=-1)
  ### ERROR ### Please RESET the board ###

Problem is with appended DTB. When CONFIG_SYS_MPC85XX_NO_RESETVEC is set
U-Boot binary image without DTB ends immediately after the .u_boot_list
section. At this position is defined _end symbol at which U-Boot expects
start of the appended DTB.

Problem is that after .u_boot_list section are in linker script defined
another sections with 256 byte long padding which are completely empty.
During conversion of U-Boot ELF binary to RAW binary u-boot-nodtb.bin,
objcopy removes trailing zero padding and therefore DTB is appended at
wrong position.

Changing alignment from 256 bytes to 4 bytes fixes this issue. And appended
DTB is finally at he correct position. With this fix U-Boot on P2020 with
CONFIG_OF_SEPARATE option starts working again.

Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Priyanka Jain <priyanka.jain@nxp.com>