[sw, dif_plic] Reset the PLIC peripheral on initialisation
[sw, dif_plic] Reset the PLIC peripheral on initialisation

Resetting a peripheral on initialisation ensures that there is no
implicit configuration, for example, from boot_rom after jump to
flash.

Signed-off-by: Silvestrs Timofejevs <5a6466a3751cccdb6ee413d1e87698ed8baaba81@lowrisc.org>

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