riscv: Unify SBI call implementations under arch/riscv/

Note that currently, traps are only handled by the trap handler
installed in the bootblock. The romstage and ramstage don't override it.

TEST=Booted emulation/spike-qemu and lowrisc/nexys4ddr with a linux
     payload. It worked as much as before (Linux didn't boot, but it
     made some successful SBI calls)

Change-Id: Icce96ab3f41ae0f34bd86e30f9ff17c30317854e
Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Reviewed-on: https://review.coreboot.org/17057
Tested-by: build bot (Jenkins)
Reviewed-by: Martin Roth <martinroth@google.com>
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>
11 files changed