soc: nordic: vpr: remove enabling `MSTATUS.MIE` in boot time

Interrupts should not be enabled this early in boot time.
Driver initializations expect IRQs not to arrive when
setting up HW.
Remove enabling `MSTATUS.MIE` in `__start`.
It will be enabled when main thread is switched to,
as threads by default start with enabled `MSTATUS.MIE`.

Signed-off-by: Marcin Szymczyk <marcin.szymczyk@nordicsemi.no>