lib: Move software interrupt enablement from firmware to lib

The secondary hart waits for an IPI signal from the boot hart to
executing boot code (hot boot). As a result, software generated
interrupts have to be enabled for secondary harts before waiting for
the boot hart boot completion IPI signal.

Enabling software generated interrupts (IPI) can be done independently
of the firmware code and moved to libsbi code so that the different
firmware do not have to implement this.

Signed-off-by: Atish Patra <atish.patra@wdc.com>