feat(hcx): add build option to enable FEAT_HCX

FEAT_HCX adds the extended hypervisor configuration register (HCRX_EL2)
and access to this register must be explicitly enabled through the
SCR_EL3.HXEn bit.  This patch adds a new build flag ENABLE_FEAT_HCX to
allow the register to be accessed from EL2.

Signed-off-by: John Powell <john.powell@arm.com>
Change-Id: Ibb36ad90622f1dc857adab4b0d4d7a89456a522b
diff --git a/docs/getting_started/build-options.rst b/docs/getting_started/build-options.rst
index 115b2b2..0ec10f5 100644
--- a/docs/getting_started/build-options.rst
+++ b/docs/getting_started/build-options.rst
@@ -235,6 +235,10 @@
    builds, but this behaviour can be overridden in each platform's Makefile or
    in the build command line.
 
+-  ``ENABLE_FEAT_HCX``: This option sets the bit SCR_EL3.HXEn in EL3 to allow
+   access to HCRX_EL2 (extended hypervisor control register) from EL2 as well as
+   adding HCRX_EL2 to the EL2 context save/restore operations.
+
 -  ``ENABLE_LTO``: Boolean option to enable Link Time Optimization (LTO)
    support in GCC for TF-A. This option is currently only supported for
    AArch64. Default is 0.