target/arm: Implement FEAT_NV2 redirection of sysregs to RAM

target/arm: Implement FEAT_NV2 redirection of sysregs to RAM

FEAT_NV2 requires that when HCR_EL2.{NV,NV2} == 0b11 then accesses by
EL1 to certain system registers are redirected to RAM.  The full list
of affected registers is in the table in rule R_CSRPQ in the Arm ARM.
The registers may be normally accessible at EL1 (like ACTLR_EL1), or
normally UNDEF at EL1 (like HCR_EL2).  Some registers redirect to RAM
only when HCR_EL2.NV1 is 0, and some only when HCR_EL2.NV1 is 1;
others trap in both cases.

Add the infrastructure for identifying which registers should be
redirected and turning them into memory accesses.

This code does not set the correct syndrome or arrange for the
exception to be taken to the correct target EL if the access via
VNCR_EL2 faults; we will do that in the next commit.

Subsequent commits will mark up the relevant regdefs to set their
nv2_redirect_offset, and if relevant one of the two flags which
indicates that the redirect happens only for a particular value of
HCR_EL2.NV1.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Tested-by: Miguel Luis <miguel.luis@oracle.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>