riscv: qemu: spl: Fix booting Linux kernel with OpenSBI 1.0+

Since OpenSBI commit bf3ef53bb7f5 ("firmware: Enable FW_PIC by default"),
OpenSBI runs directly at the load address without any code movement.
This causes the SPL version of QEMU 'virt' U-Boot does not boot Linux
kernel anymore. In that case, OpenSBI is loaded and runs at 0x81000000,
and it creates a 512KiB PMP window from that address. When booting
the Linux kernel, moving kernel to its linking address 0x80200000
overlaps the PMP window, and a PMP access failure is raised.

Update SPL_OPENSBI_LOAD_ADDR to load OpenSBI to a safe address.

Reported-by: Yangjie Zhang <pyjmstr@gmail.com>
Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Yangjie Zhang <pyjmstr@gmail.com>
Reviewed-by: Rick Chen <rick@andestech.com>