Aarch64: Add support for FEAT_PANx extensions

Aarch64: Add support for FEAT_PANx extensions

This patch provides the changes listed below:
- Adds new bit fields definitions for SCTLR_EL1/2 registers
- Corrects the name of SCTLR_EL1/2.[20] bit field from
SCTLR_UWXN_BIT to SCTLR_TSCXT_BIT
- Adds FEAT_PANx bit field definitions and their possible
values for ID_AA64MMFR1_EL1 register.
- Adds setting of SCTLR_EL1.SPAN bit to preserve PSTATE.PAN
on taking an exception to EL1 in spm_sp_setup() function
(services\std_svc\spm_mm\spm_mm_setup.c)

Change-Id: If51f20e7995c649126a7728a4d0867041fdade19
Signed-off-by: Alexei Fedorov <Alexei.Fedorov@arm.com>