sam9x60-curiosity: migrate Boot LED setup to use /options/u-boot/boot-led

sam9x60-curiosity: migrate Boot LED setup to use /options/u-boot/boot-led

This board is one of the last users of /config/u-boot,boot-led property
which is a U-Boot property out of the DT spec.

Let's migrate it to use the in-spec /options/u-boot/boot-led property.
When enabling LED_BOOT, U-Boot proper will lit the LED right before
entering the main loop, so nothing needs to be done in board files.

As explained in the commit adding support for this u-boot,boot-led
property, let's keep backward compatibility in case LED_BOOT isn't
selected.

Note that this is not tested as I do not own this device.

Cc: Alexander Dahl <ada@thorsis.com>
Signed-off-by: Quentin Schulz <quentin.schulz@cherry.de>
Tested-by: Alexander Dahl <ada@thorsis.com>