refactor(context mgmt): refactor initialization of EL1 context registers

When SPMC is present at S-EL2, EL1 context registers don't need to be
initialized for Secure state. This patch makes sure that EL1 context
registers are initialized only for Non-secure state, and when SPMC is
not present at S-EL2

Signed-off-by: Zelalem Aweke <zelalem.aweke@arm.com>
Change-Id: I4a60b258c31ce5f6472a243e2687159cc495259b