This commit introduces two separate default configuration files to manage
the build differences between DVR and Video Server Products.

- Add configs/hisilicon_hi3516dv300_dvr_defconfig: Default configuration for DVR
- Add configs/hisilicon_hi3516dv300_vidsrv_defconfig: Default configuration for Video Server

This enables building each product with:
  make hisilicon_hi3516dv300_dvr_defconfig
  make hisilicon_hi3516dv300_vidsrv_defconfig