docs/system/arm: Add FEAT_HCX to list of emulated features

docs/system/arm: Add FEAT_HCX to list of emulated features

In commit 5814d587fe861fe9 we added support for emulating
FEAT_HCX (Support for the HCRX_EL2 register). However we
forgot to add it to the list in emulated.rst. Correct the
omission.

Fixes: 5814d587fe861fe9 ("target/arm: Enable FEAT_HCX for -cpu max")
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20220520084320.424166-1-peter.maydell@linaro.org