ARM: bootm: Allow booting in secure mode on hyp capable systems

Older Linux kernels will not properly boot in hype mode, add support for a
bootm_boot_mode environment variable, which when set to "sec" will cause
u-boot to boot in secure mode even when build with non-sec (and hyp) support.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Marc Zyngier <marc.zyngier@arm.com>
Acked-by: Siarhei Siamashka <siarhei.siamashka@gmail.com>