STM32H7 does not have the a functioning Timer driver

Related to https://github.com/apache/incubator-nuttx-apps/issues/834

I was trying to determine a way to use the timer with my NUCLEOH743ZI2 board. Following the guidelines provided in the above issue, I was still experiencing some problems. I did a deep dive into the what the ultimate cause of the problem was. This is as follows:

When navigating to `nuttx -> arc -> arm -> src -> stm32` This folder contains a `stm32_tim_lowerhalf.c` file that defines the function `stm32_timer_initialize` which we need in order to get any timer up and operating. However in the `stm32h7` folder found in the same location as the `stm32` folder I noticed that the `stm32_tim.h` file defines it, but the `stm32_tim.c` file does not integrate it. Additionally, the `stm32_tim_lowerhalf.c` file does not exist.

Ultimately, this cause the `startup.c` file to fail on calling the `stm32_timer_initialize` function as it does not think that it exists. I am unsure how to go about using timers with the `H7` board.

Any thoughts, or feedback would be great! Thank you for your time!