powerpc/mpc85xx: explicit cast the SDRAM size to type phys_size_t

To avoid sign extension problem, use explicit casting to cast
the SDRAM size to type phys_size_t, or else, if the SDRAM size
is 2G(0x80000000), it will be extended to 0xffffffff80000000
when phys_size_t is type 'unsigned long long'.

Signed-off-by: Mingkai Hu <Mingkai.Hu@freescale.com>
Signed-off-by: Andy Fleming <afleming@freescale.com>