N6 IC Selection Fix

[jake-taf]

The CPU/SYS clocks from init_clocks has a bug due to a incorrect enum generation, causing HSI clocks to be used instead of PLL 3/4

`rcc_n6.rs generated from stm32-data incorrectly uses the following for IC selection:

pub enum Icsel {
    #[doc = "pll1_ck is selected (default after reset)."]
    PLL1 = 0x0,
    #[doc = "pll2_ck is selected."]
    PLL2 = 0x01,
    #[doc = "hsi_ck = hsi_osc_ck / 4."]
    HSI_OSC_DIV4 = 0x02,
    #[doc = "hsi_ck = hsi_osc_ck / 8."]
    HSI_OSC_DIV8 = 0x03,
}

when it should be this (default is different for different IC, so the doc was also misleading).

pub enum Icsel {
    #[doc = "pll1_ck is selected."]
    PLL1 = 0x0,
    #[doc = "pll2_ck is selected."]
    PLL2 = 0x01,
    #[doc = "pll3_ck is selected."]
    PLL3 = 0x02,
    #[doc = "pll4_ck is selected."]
    PLL3 = 0x03,
}

Caution

This relies on another PR to work, so those must be merged around the same time
This issue is fixed in embassy-rs/stm32-data#727