arch/riscv: Only execute on hart 0 for now

Only execute coreboot on hart 0 until synchronisation between hart's is ready.
Change-Id: I2181e79572fbb9cc7bee39a3c2298c0dae6c1658
Signed-off-by: Philipp Hug <philipp@hug.cx>
Reviewed-on: https://review.coreboot.org/28605
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>
1 file changed