boot/arm-trusted-firmware: support 32bit Arm targets

boot/arm-trusted-firmware: support 32bit Arm targets

This change allows one to build the TF-A (Trusted Firmware A) for
32bit architectures Armv7 and Armv8 platforms. TF-A supports these
configuration since release v1.5.

Change dependency on BR2_aarch64 into a dependency on either
BR2_ARM_CPU_ARMV8A/BR2_ARM_CPU_ARMV7A to better define
the target architectures supported by the package.

Set package configuration directive ARM_ARCH_MAJOR according to
BR2_ARM_CPU_ARMV8A/BR2_ARM_CPU_ARMV7A. Set package configuration
directive ARCH based on BR2_aarch64/BR2_arm.

Signed-off-by: Etienne Carriere <etienne.carriere@linaro.org>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>