imx: Use 'const char', not 'char const'

imx: Use 'const char', not 'char const'

'char const' means the same thing as 'const char', but we
use the former in only a handful of places and we use the
latter over six thousand times. Switch the imx reg_name()
functions to bring them in line with everything else.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>