xen/arm: Rework HSCTLR_BASE

xen/arm: Rework HSCTLR_BASE

The current value of HSCTLR_BASE for Arm64 is pretty wrong. It would
actually turn on SCTLR_EL2.nAA (bit 6) on hardware implementing
ARMv8.4-LSE.

Furthermore, the documentation of what is cleared/set in SCTLR_EL2 is
also not correct and looks like to be a verbatim copy from Arm32.

HSCTLR_BASE is replaced with a bunch of per-architecture new defines
helping to understand better what is the initial value for
SCTLR_EL2/HSCTLR.

Note the defines *_CLEAR are only used to check the state of each bits
are known.

Lastly, the documentation is dropped from arm{32,64}/head.S as it would
be pretty easy to get out-of-sync with the definitions.

Signed-off-by: Julien Grall <julien.grall@arm.com>
Reviewed-by: Andrii Anisov <andrii.anisov@epam.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>