powerpc/85xx: Introduce CONFIG_SYS_EXTRA_ENV_RELOC

Some systems need to relocate the env_addr pointer early because the
location it points to will get invalidated before env_relocate is
called.  One example is on systems that might use a L2 or L3 cache
in SRAM mode and initialize that cache from SRAM mode back to being
a cache in cpu_init_r.

We set this on the 85xx boards that have support for NAND, SPI, or
SDHC/MMC boot support as they use a secondary cache in SRAM mode and
need the env_addr pointer relocated since we change from SRAM to normal
cache mode in cpu_init_r.

Also removed CONFIG_SYS_SPL as its not used anywhere.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>