arm: move CONFIG_TFABOOT to generic Kconfig

arm: move CONFIG_TFABOOT to generic Kconfig

Currently, CONFIG_TFABOOT is located in armv8/fsl-layerscape Kconfig,
but it will be also useful for other targets if some additional
configuration are necessary.
So move it to arch/arm/Kconfig.

Please note that CONFIG_TFABOOT still depends on
CONFIG_ARCH_SUPPORT_TFABOOT and so the menu won't come up
if any target doesn't need its own customization for TF-A boot.
This will maintain the compatibility.

Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
Cc: Rajesh Bhagat <rajesh.bhagat@nxp.com>
Cc: York Sun <york.sun@nxp.com>
Cc: Prabhakar Kushwaha <prabhakar.kushwaha@nxp.com>
Cc: Priyanka Jain <priyanka.jain@nxp.com>
Cc: Sriram Dash <sriram.dash@nxp.com>
Cc: Hou Zhiqiang <Zhiqiang.Hou@nxp.com>
Cc: Peng Ma <peng.ma@nxp.com>
Cc: Yuantian Tang <andy.tang@nxp.com>
Cc: Pankit Garg <pankit.garg@nxp.com>