rockchip: move CONFIG_ENV_SIZE and CONFIG_ENV_OFFSET to Kconfig

This commit adds ENV_SIZE and ENV_OFFSET configuration items for
ARCH_ROCKCHIP, but keeps these non-visible (i.e. not prompt is given).
With these new items present, the configuration from the header files
is moved to Kconfig.

Keeping these non-visible is necessary to have the possibility to
select new default values if CONFIG_IS_IN_* is changed (interactively
or with oldconfig). Otherwise it will always be set to a previous
value if used with a prompt.  As an example if we do a defconfig with
CONFIG_IS_IN_MMC and change it to CONFIG_IS_IN_SPI_FLASH via
menuconfig, ENV_SIZE and ENV_OFFSET will not be changed to the correct
values as defconfig will already have set them to the default values
of CONFIG_IS_IN_MMC in .config.

Signed-off-by: Klaus Goger <klaus.goger@theobroma-systems.com>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>