powerpc/85xx: Fix sram_offset parameter type

The sram_offset parameter represents a physical address and should be of
type phys_addr_t. As part of this fix, the extraction of sram_params is
being cleaned-up and fixed.

This patch fixes now the case when the offset value of 0xfff00000 was being
rejected by the driver (returning -EINVAL), although this is a valid offset
value.

Signed-off-by: Tang Yuantian <Yuantian.Tang@freescale.com>
Signed-off-by: Claudiu Manoil <claudiu.manoil@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>