env: Enable SPI flash env for rockchip

Most of the SPI flash devices in rockchip are 16MiB size.

So, keeping U-Boot proper offset start from 128MiB with 1MiB
size and then start env of 8KiB would be a compatible location
between all variants of flash sizes.

This patch add env start from 0x14000 with a size of 8KiB.

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