Add patches for ATmega8

[Rutherther]

This PR adds patches for ATmega8 needed to implement ATMega8 to avr-hal easily.

Unfortunately I had to edit the vendor file, because there is a field WGM2 consisting of WGM21 and WGM20 that are not directly next to each other. Any idea on how to patch this correctly?

[Rutherther]

Hm. I am looking at the generated code and it doesn't seem that having a field on positions not directly to each others is even possible now.

Can a support for that be added and would that be desirable? Or should I just remove that and WGM2 will have to be configured directly without an enum?

[Rutherther]

ATmega8 uses the same register for UCSRC and UBRRH, the way to differentiate between them is that when setting UCSRC, you have to make URSEL (MSb) one. Is there a possibility to handle that automatically? Currently the generated code contains functions in usart.rs instead of fields

    #[doc = "0x17 - USART Control and Status Register C"]
    #[inline(always)]
    pub fn ucsrc(&self) -> &crate::Reg<ucsrc::UCSRC_SPEC> {
        unsafe {
            &*(((self as *const Self) as *const u8).add(23usize)
                as *const crate::Reg<ucsrc::UCSRC_SPEC>)
        }
    }
    #[doc = "0x17 - USART Baud Rate Register Hight Byte"]
    #[inline(always)]
    pub fn ubrrh(&self) -> &crate::Reg<ubrrh::UBRRH_SPEC> {
        unsafe {
            &*(((self as *const Self) as *const u8).add(23usize)
                as *const crate::Reg<ubrrh::UBRRH_SPEC>)
        }
    }

But it does not seem that it sets MSB to one automatically, that's understandable as there is nothing about that in the vendor files nor in patches.

[Rutherther]

Alright, I thought these problems could be solved, that was the reason it was a draft. I think it may be merged now.