modules: trusted-firmware-a: Disable AArch32 context for ARMv9-A

ARMv9-A is AArch64-only and does not support AArch32 execution states
at EL1/EL2/EL3. Configure TF-A build to disable AArch32 register
context (CTX_INCLUDE_AARCH32_REGS=0) when building for ARMv9-A platforms.

This fixes TF-A boot failures on ARMv9-A non-secure variants where
TF-A would panic with:
  ERROR: EL1 supports AArch64-only. Please set build flag
         CTX_INCLUDE_AARCH32_REGS = 0

Signed-off-by: Nicolas Pitre <npitre@baylibre.com>