docs/system/arm/emulation.rst: Report FEAT_GTG support

docs/system/arm/emulation.rst: Report FEAT_GTG support

FEAT_GTG is a change tho the ID register ID_AA64MMFR0_EL1 so that it
can report a different set of supported granule (page) sizes for
stage 1 and stage 2 translation tables.  As of commit c20281b2a5048
we already report the granule sizes that way for '-cpu max', and now
we also correctly make attempts to use unimplemented granule sizes
fail, so we can report the support of the feature in the
documentation.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 20221003162315.2833797-4-peter.maydell@linaro.org