When pointer authentication is enabled, the LR value saved on the stack
contains a Pointer Authentication Code (PAC). It must be stripped to
retrieve the return address.

The PAC field is stored on the high bits of the address and defined as:
- PAC field = Xn[54:bottom_PAC_bit], when address tagging is used.
- PAC field = Xn[63:56, 54:bottom_PAC_bit], without address tagging.

With bottom_PAC_bit = 64 - TCR_ELx.TnSZ

Change-Id: I21d804e58200dfeca1da4c2554690bed5d191936
Signed-off-by: Louis Mayencourt <louis.mayencourt@arm.com>