am335x: BeagleBones enable CONFIG_AUTOBOOT_KEYED

On startup, the BeagleBone family of boards can occationally halt at
the U-Boot prompt.  Due to U-Boot receiving random data on the usart.

Migrate BeagleBone based boards to use CONFIG_AUTOBOOT_KEYED, user
will now have to enter the <SPACE> key to get to U-Boot prompt

Signed-off-by: Robert Nelson <robertcnelson@gmail.com>
CC: Craig McQueen <craig.mcqueen@innerrange.com>
CC: Tom Rini <trini@konsulko.com>
Reviewed-by: Tom Rini <trini@konsulko.com>