Clarify STM32U0 delay-default frequency usage

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Co-authored-by: HiFiPhile <4375114+HiFiPhile@users.noreply.github.com>

Author: Copilot

src/portable/st/stm32_fsdev/fsdev_stm32.h

@@ -146,6 +146,7 @@
   #elif CFG_TUSB_MCU == OPT_MCU_STM32U3
     #define FSDEV_STM32_CPU_MHZ 96U
   #elif CFG_TUSB_MCU == OPT_MCU_STM32U0
+    // Used by STM32 FSDEV PMA delay defaults as a conservative max CPU frequency.
     #define FSDEV_STM32_CPU_MHZ 56U
   #elif CFG_TUSB_MCU == OPT_MCU_STM32G0
     #define FSDEV_STM32_CPU_MHZ 64U