mach-omap2: Cleanup secure boot media generation

Currently all secure media types of SPL are generated for all platforms,
all platforms do not need all types, only generate the media types valid
for each platform.

Signed-off-by: Andrew F. Davis <afd@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Tested-by: Lokesh Vutla <lokeshvutla@ti.com>