imx: fix USB boot mode detection for i.MX 6UL and 6ULL

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>