refactor(cm): couple el2 registers with dependent feature flags

Currently the EL2 part of the context structure (el2_sysregs_t), is
mostly feature dependent.

For instance, CTX_HCRX_EL2 is only needed when FEAT_HCX
(ENABLE_FEAT_HCX=1) is set, but the entry is unconditionally added
in the EL2 context structure and thereby consuming memory even in
build configurations where FEAT_HCX is disabled.

Henceforth, all such context entries should be coupled/tied with
their respective feature enables and be optimized away when unused.
This would reduce the context memory allocation for platforms, that
dont enable/support all the architectural features at once.

Further, converting the assembly context-offset entries into
a c structure relies on garbage collection of the linker
removing unreferenced structures from memory, as well as aiding
in readability and future maintenance.

Change-Id: I0cf49498ee3033cb6f3ee3810331121b26627783
Signed-off-by: Jayanth Dodderi Chidanand <jayanthdodderi.chidanand@arm.com>