interrupt_controller: added support for the SiFive Freedom E310 PLIC driver

The SiFive Freedom E310 Platform Level Interrupt Controller (PLIC) follows
the riscv PLIC specification as defined in the riscv privilege
architecture specification.

It provides implementation for the riscv PLIC APIs as
required by the riscv-privilege SOC Family for SOCs providing
support for the RISCV_HAS_PLIC config.

Change-Id: I95d02edb53deeccd91e490776e8e1dbfb82d235f
Signed-off-by: Jean-Paul Etienne <fractalclone@gmail.com>