Add the reserved boot mode used in the bmode command for i.MX 6UL
and 6ULL as introduced in commit 3fd95790 ("imx: mx6ull: fix USB
bmode for i.MX 6UL and 6ULL").

Also replace BMODE_UART with BMODE_RESERVED, which is more appropriate.
Commit 96aac843 ("imx: Use IMX6_BMODE_* macros instead of numericals")
added macros for boot modes, in the process the reserved boot mode got
named BMODE_UART. We use the reserved boot mode in the bmode command to
let the boot ROM enter serial downloader recovery mode. But this is only
a side effect, the actual boot mode is reserved...

Signed-off-by: Stefan Agner <stefan.agner@toradex.com>