doc: rockchip: puma: update build and flash instructions

Long gone is the time a custom TF-A was needed for Puma, upstream TF-A
works just fine now.

The flashing instructions are updated to match how newer rkdeveloptool
and rkbin work.

Finally, rkbin provides a way to flash SPI via USB OTG interface so
let's document that.

Cc: Quentin Schulz <foss+u-boot@0leil.net>
Signed-off-by: Quentin Schulz <quentin.schulz@theobroma-systems.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>