board: puma: Get bl31.bin via BL31 and rk3399m0.bin via PMUM0

board: puma: Get bl31.bin via BL31 and rk3399m0.bin via PMUM0

Right now puma rk3399 board need to copy bl31-rk3399.bin and
rk3399m0.bin into u-boot source directory to make use of building
u-boot.itb.

So, add environment variable
- BL31 for bl31.bin (instead of bl31-rk3399.bin to compatible with other
  platform BL31 env)
- PMUM0 for rk3399m0.bin

If the builds are not exporting BL31, PMUM0 env, the fit_spl_atf.sh will
notify with warning about which document to refer for more information
like this:

 WARNING: BL31 file bl31.bin NOT found, resulting binary is non-functional
 Please read Building section in doc/README.rockchip
 WARNING: PMUM0 file rk3399m0.bin NOT found, resulting binary is non-functional
 Please read Building section in doc/README.rockchip

Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>