nanopi-r6c/r6s: recover vendor u-boot

- the vendor uboot stuff was already in, by Efe a few months ago; doesn't have nvme
- update defconfig to drop optee stuff so it works again
- does not change board file, thus is absolutely not used anywhere

patch/u-boot/legacy/u-boot-radxa-rk35xx/defconfig/nanopi-r6s-rk3588s_defconfig

@@ -156,7 +156,6 @@ CONFIG_REGULATOR_PWM=y
 CONFIG_DM_REGULATOR_FIXED=y
 CONFIG_DM_REGULATOR_GPIO=y
 CONFIG_REGULATOR_RK860X=y
-CONFIG_REGULATOR_RK806=y
 CONFIG_PWM_ROCKCHIP=y
 CONFIG_RAM=y
 CONFIG_SPL_RAM=y
@@ -212,6 +211,3 @@ CONFIG_AVB_LIBAVB_AB=y
 CONFIG_AVB_LIBAVB_ATX=y
 CONFIG_AVB_LIBAVB_USER=y
 CONFIG_RK_AVB_LIBAVB_USER=y
-CONFIG_OPTEE_CLIENT=y
-CONFIG_OPTEE_V2=y
-CONFIG_OPTEE_ALWAYS_USE_SECURITY_PARTITION=y