soc: cva6: Fix CONFIG_MAX_IRQ_PER_AGGREGATOR

The original commit uses the incorrect value 42 for
CONFIG_MAX_IRQ_PER_AGGREGATOR for the cva6 family of SoCs,
which is the total number of IRQs in the system.
This commit corrects this to 30, the number of IRQs for the PLIC.

Signed-off-by: Eric Ackermann <eric.ackermann@cispa.de>