Implement hypervisor CSRs read/write

We add newly defined hypervisor CSRs and allow M/HS-mode to access
these CSRs. The MRET, SRET, ECALL and WFI instructions have also
been updated so that virt-to-novirt switch and exception cause is
based on HART virtualization state.

Subsequent patches will implement two-stage page tables, HFENCE
instructions and HSV/HLV instructions.

Signed-off-by: Anup Patel <anup.patel@wdc.com>