fix(morello): avoid capability tag fault on data access

TF-A runtime service at EL3 switches the stack pointer from SP_EL3
to SP_EL0. This creates a capability tag fault when the DDC_EL0 is
zeroed out (purecap user space) as any data accesses computes
tag/permission with DDC_EL0 value when SpSel is 0 and when EL3 is
in hybrid mode.

As a workaround, this patch creates a per cpu context variable
to store DDC_EL0 value so that when EL3 runtime is entered DDC_EL0
is saved on to stack. DDC_EL3 is then copied into DDC_EL0 after
switching SP to SP_EL0. Once the runtime finishes, during el3_exit,
the saved DDC_EL0 is restored from stack.

Signed-off-by: Selvarasu Ganesan <selvarasu.ganesan@arm.com>
Signed-off-by: Manoj Kumar <manoj.kumar3@arm.com>
Change-Id: I4e4010f0e20913cb4e35b58fb49a177bdf26feb1