am62x: bootloader: add build dependencies
These are documented in [1] but not here.
Add them here as well. Note that meson is not part of this since meson is not used for TI bootloaders.
[1] https://mediatek.gitlab.io/aiot/rita/device/mediatek/board/bootloader/bootloaders.html#building
Signed-off-by: Mattijs Korpershoek mkorpershoek@baylibre.com