lib: irqchip/plic: Factor out a context init function

This simplifies both the callers and the callees by removing duplicated
code and consolidating the error handling. It also fixes two bugs in the
process:
  1) ie_words was one too large when plic->num_src was a multiple of 32.
  2) plic_set_ie takes a 32-bit mask, not a Boolean value, so the FPGA
     platforms previously only enabled one out of every 32 interrupts.

Reviewed-by: Anup Patel <anup@brainfault.org>
Signed-off-by: Samuel Holland <samuel@sholland.org>