binman: cosmetic: refactor `nxp_imx8mcst' etype code

Simplify code and conform to the style guide used in the project by
making the following changes:
* Capitalize global constants
* Use single quotes for multiline strings (except docstrings)
* Fix line width to 79 cols
* Use f-string instead of formatting a regular string or using a
  complicated concatenation
* Move common suffix used in keys to a global variable "KEY_NAME"
  to reduce the likelihood of typos and making future changes
  easier

Signed-off-by: Brian Ruley <brian.ruley@gehealthcare.com>
Cc: Marek Vasut <marex@denx.de>