boot/arm-trusted-firmware: add optional host-dtc dependency

boot/arm-trusted-firmware: add optional host-dtc dependency

Some ATF configurations, such as the ones for the STM32MP1 processor
family, require DTC during the build as Device Tree files are
used. Without dtc, the build fails:

/bin/sh: 1: dtc: not found
dtc version too old (), you need at least version 1.4.4
plat/st/stm32mp1/platform.mk:239: recipe for target 'check_dtc_version' failed
make[1]: *** [check_dtc_version] Error 1

To solve this, this commit implements a
BR2_TARGET_ARM_TRUSTED_FIRMWARE_NEEDS_DTC option, in a way that mimics
the BR2_TARGET_UBOOT_NEEDS_DTC option we already have for the U-Boot
package.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>