arm: am4x: fix build dependency for secure devices

Commit e29878fc introduces an undesired dependency on CONFIG_SPL_LOAD_FIT
when building U-Boot for AM43xx high-security (HS) devices that causes the
build to break when that option is not active. Fix this issue by only
building the u-boot_HS.img target when building U-Boot into an actual
FIT image.
Signed-off-by: NAndreas Dannenberg <dannenberg@ti.com>
Reviewed-by: NTom Rini <trini@konsulko.com>

arch/arm/cpu/armv7/am33xx/config.mk

@@ -26,7 +26,7 @@ endif
 else
 ifeq ($(CONFIG_TI_SECURE_DEVICE),y)
 ALL-$(CONFIG_QSPI_BOOT) += u-boot_HS_XIP_X-LOADER
-ALL-y	+= u-boot_HS.img
+ALL-$(CONFIG_SPL_LOAD_FIT) += u-boot_HS.img
 endif
 ALL-y	+= u-boot.img
 endif