doc: rockchip: Update build instruction for rk3308

After commit d8765e24 ("Enable building of u-boot.itb
on Rockchip platform"), u-boot.itb will automatically
generated by "make all" command, manually command
"make u-boot.itb" is no longer needed.

Signed-off-by: Andy Yan <andy.yan@rock-chips.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>

doc/README.rockchip

@@ -108,7 +108,6 @@ For example:
      => export BL31=/path/to/rkbin/bin/rk33/rk3308_bl31_v2.22.elf
      => make roc-rk3308-cc_defconfig
      => make CROSS_COMPILE=aarch64-linux-gnu- all
-     => make CROSS_COMPILE=aarch64-linux-gnu- u-boot.itb
      => ./tools/mkimage -n rk3308 -T rksd -d /path/to/rkbin/bin/rk33/rk3308_ddr_589MHz_uart2_m0_v1.26.bin idbloader.img
      => cat spl/u-boot-spl.bin  >> idbloader.img