ARM: imx: imx8m: env: Switch to arch_env_get_location()

Implement arch_env_get_location() instead of env_get_location(), so that
the env_get_location() can be implemented on board level and override the
arch_env_get_location() architecture defaults.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Adam Ford <aford173@gmail.com>
Cc: Fabio Estevam <festevam@denx.de>
Cc: Marek Behún <marek.behun@nic.cz>
Cc: Peng Fan <peng.fan@nxp.com>
Cc: Simon Glass <sjg@chromium.org>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Tim Harvey <tharvey@gateworks.com>
Cc: Tom Rini <trini@konsulko.com>
Cc: Tommaso Merciai <tomm.merciai@gmail.com>
Cc: Vladimir Oltean <vladimir.oltean@nxp.com>