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>
diff --git a/doc/README.rockchip b/doc/README.rockchip
index 4efd9c1..d78d993 100644
--- a/doc/README.rockchip
+++ b/doc/README.rockchip
@@ -138,8 +138,8 @@
 	=> cd arm-trusted-firmware
 	=> make CROSS_COMPILE=aarch64-linux-gnu- PLAT=rk3399 bl31
 
-	(copy bl31.bin into U-Boot root dir)
-	=> cp build/rk3399/release/bl31/bl31.bin /path/to/u-boot/bl31-rk3399.bin
+	(export bl31.bin)
+	=> export BL31=/path/to/arm-trusted-firmware/build/rk3399/release/bl31/bl31.bin
 
      For rest of rk3399 boards.
 
@@ -163,8 +163,8 @@
      (export cross compiler path for Cortex-M0 PMU)
      => make CROSS_COMPILE=arm-cortex_m0-eabi-
 
-     (copy rk3399m0.bin into U-Boot root dir)
-     => cp rk3399m0.bin /path/to/u-boot
+     (export rk3399m0.bin)
+     => export PMUM0=/path/to/rk3399-cortex-m0/rk3399m0.bin
 
    - Compile U-Boot