Add support for mtvec direct mode

[tomeroberts]

Some SW use cases would prefer to have support for "direct" interrupt mode rather than "vectored" mode in the mtvec csr (support for FreeRTOS and for boot ROMs e.g. in OpenTitan).

The rtl change is trivial but the impact on DV needs to be investigated.

[alphan]

We should update sw/device/silicon_creator/mask_rom/mask_rom_start.S to use this instead of vectored mode once this is implemented.