The ability to obtain offset values for the modifiable bit fields of registers

To be able to manipulation of peripheral registers through bit-banding, the values of the offset of the modifiable bit fields in the registers are required.

For example, there are functions for bit-banding here: https://docs.rs/stm32f1xx-hal/0.9.0/stm32f1xx_hal/bb/index.html

Implementation variant:

pub mod perih_name {
   ...
   pub mod reg_name {
      ...
      pub const field_name_OFFSET: u8 = ...;
      ...
   }
   ...
}

Example of an implementation variant (stm32f1):

pub mod rcc {
   ...
   pub mod apb2enr {
      ...
      pub const USART1EN_OFFSET: u8 = 14;
      ...
      impl<'a> USART1EN_W<'a> {
         ...
         pub fn bit(self, value: bool) -> &'a mut W {
            self.w.bits = (self.w.bits & !(0x01 << USART1EN_OFFSET)) | ((value as u32 & 0x01) << USART1EN_OFFSET);
            self.w
         }
      }
      ...
      impl R {
         ...
         pub fn usart1en(&self) -> USART1EN_R {
            USART1EN_R::new(((self.bits >> USART1EN_OFFSET) & 0x01) != 0)
         }
         ...
      }
      ...
   }
   ...
}

Usage example:

unsafe {
   use stm32f1xx_hal::{self as hal, pac};
   hal::bb::set(&(*pac::RCC::PTR).apb2enr, pac::rcc::apb2enr::USART1EN_OFFSET);
}

Start by comparing the requested generated register modules with the stm32f1 example and the bit-banding functions documented in stm32f1xx-hal. Determine how the generated field offset values should be exposed and verify that the usage pattern shown in the issue is supported.