BL2_AT_EL3: Enable pointer authentication support

BL2_AT_EL3: Enable pointer authentication support

The size increase after enabling options related to ARMv8.3-PAuth is:

+----------------------------+-------+-------+-------+--------+
|                            |  text |  bss  |  data | rodata |
+----------------------------+-------+-------+-------+--------+
| CTX_INCLUDE_PAUTH_REGS = 1 |   +44 |   +0  |   +0  |   +0   |
|                            |  0.2% |       |       |        |
+----------------------------+-------+-------+-------+--------+
| ENABLE_PAUTH = 1           |  +712 |   +0  |  +16  |   +0   |
|                            |  3.1% |       |  0.9% |        |
+----------------------------+-------+-------+-------+--------+

The results are valid for the following build configuration:

    make PLAT=fvp SPD=tspd DEBUG=1 \
    BL2_AT_EL3=1                   \
    CTX_INCLUDE_PAUTH_REGS=1       \
    ENABLE_PAUTH=1

Change-Id: I1c0616e7dea30962a92b4fd113428bc30a018320
Signed-off-by: Antonio Nino Diaz <[email protected]>