drivers: intc: plic: Fix memory-mapped register offset calculation

Previously, the PLIC's registers were accessed through uint32_t *,
so all calculated offsets were effectively multiplied by
sizeof(uint32_t). Do the same manuallly now that we have
mem_addr_t/sys_read32.

Signed-off-by: Piotr Wojnarowski <pwojnarowski@antmicro.com>