arm: imx: hab: Rename is_hab_enabled imx_hab_is_enabled

Understanding if the HAB is enabled is something that we want to
interrogate and report on outside of the HAB layer. First step to that is
renaming the relevant function to match the previously introduced external
naming convention imx_hab_function()

The name imx_hab_is_hab_enabled() is a tautology. A more logical name is
imx_hab_is_enabled().

Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Fabio Estevam <fabio.estevam@nxp.com>
Cc: Peng Fan <peng.fan@nxp.com>
Cc: Albert Aribaud <albert.u.boot@aribaud.net>
Cc: Sven Ebenfeld <sven.ebenfeld@gmail.com>
Cc: George McCollister <george.mccollister@gmail.com>
Cc: Breno Matheus Lima <brenomatheus@gmail.com>
Tested-by: Breno Lima <breno.lima@nxp.com>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>