lib: el3_runtime: Fix aarch32 system registers in el2_sysregs_context
AArch64-only platforms do not implement AArch32 at EL1 and higher ELs.
In such cases the build option CTX_INCLUDE_AARCH32_REGS is set to 0.
So don't save/restore aarch32 system registers in el2_sysregs_context
save/restore routines if CTX_INCLUDE_AARCH32_REGS is set to 0.