refactor(cm): convert el1-ctx assembly offset entries to c structure

Currently the EL1 part of the context structure (el1_sysregs_t),
is coupled with feature flags reducing the context memory allocation
for platforms, that don't enable/support all the architectural
features at once.

Similar to the el2 context optimization commit-"d6af2344" this patch
further improves this section by converting the assembly context-offset
entries into a c structure. It relies on garbage collection of the
linker removing unreferenced structures from memory, as well as aiding
in readability and future maintenance. Additionally, it eliminates
the #ifs usage in 'context_mgmt.c' source file.

Change-Id: If6075931cec994bc89231241337eccc7042c5ede
Signed-off-by: Jayanth Dodderi Chidanand <jayanthdodderi.chidanand@arm.com>