arm: omap5: fix build dependency for secure devices

Commit 17c29873 introduces an undesired dependency on CONFIG_SPL_LOAD_FIT
when building U-Boot for AM57xx and DRA7xx 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: Andreas Dannenberg <dannenberg@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>