rockchip: board: puma_rk3399: update README flash instructions

Puma supports other boot sources then SD-Card. Update README to include
the required steps.

 * how to package a SPI-NOR SPL
 * how to flash eMMC with rkdeveloptool

Signed-off-by: Klaus Goger <klaus.goger@theobroma-systems.com>
Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Acked-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
1 file changed