recipes-bsp/opensbi: Disable FW_PIC

Disable FW_PIC to fix the error message:

opensbi-0.9-r0 do_package_qa: QA Issue: File /share/opensbi/lp64/generic/firmware/fw_jump.elf in package opensbi doesn't have GNU_HASH (didn't pass LDFLAGS?)

This is seen in newer versions of OpenSBI that enable FW_PIC by default.

Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>