soc/arm64: Do not allow userspace to directly access peripherals

In some arm64 boards, MMU regions are set so userspace can access all
peripherals (MT_P_RW_U_RW), including the GIC.
This seems like a mistake, which has been copy/pasted in some boards.

Change this to no userspace access by default (MT_P_RW_U_NA),
like the other boards.

Signed-off-by: Henri Xavier <datacomos@huawei.com>