powerpc/85xx: Fix signedness bug in cache-sram

sram_params.sram_size and sram_params.sram_offset were unsigned.
If get_cache_sram_size() or get_cache_sram_offset() returns error code
then it is not seen to the caller.  Made sram_size and sram_offset signed.

Signed-off-by: Vasiliy Kulikov <segooon@gmail.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>