imx: fix header inclusion guards

It seems like the header inclusion guards for some IMX related headers
were misspelled or got out of sync.

Make the preprocessor symbols for the #ifndef and #define lines the
same, so that the double inclusion protection works as expected.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Tom Rini <trini@konsulko.com>