powerpc: spl: configure SYS_MALLOC_F_LEN independently for SPL and full U-Boot

powerpc: spl: configure SYS_MALLOC_F_LEN independently for SPL and full U-Boot

Some platforms have very limited SRAM to run SPL code, so there may
not be the same amount space for a malloc pool before relocation in
the SPL stage as the normal U-Boot stage.

Make SPL and (the full) U-Boot stage use independent SYS_MALLOC_F_LEN,
so the size of pre-relocation malloc pool can be configured memory
space independently.

Signed-off-by: Andy Yan <[email protected]>
Acked-by: Philipp Tomsich <[email protected]>
Reviewed-by: Philipp Tomsich <[email protected]>
[fixed up commit-message:]
Signed-off-by: Philipp Tomsich <[email protected]>