sandbox: config: Don't use 64-bit physical memory

Sandbox uses an emulated memory map which is quite small. We don't need the
CONFIG_PHYS_64BIT option since we can address memory with a 32-bit offset
into our ram_buf.

Adjust the phys_addr_t and phys_size_t types accordingly.

Signed-off-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Hung-ying Tyan <tyanh@chromium.org>