arch: riscv: Increase IDT_LIST size

Some boards fail to build arch.interrupt.gen_isr_table_local.riscv test
since this region is too small:

riscv64-zephyr-elf/bin/ld.bfd: zephyr/zephyr_pre0.elf section `.intList'
will not fit in region `IDT_LIST'

riscv64-zephyr-elf/bin/ld.bfd: region `IDT_LIST' overflowed by 239 bytes

This is a bogus memory region, so increasing it has no effect on the
final binary size.

Issue #92194

Signed-off-by: Tom Hughes <tomhughes@chromium.org>