Air32F103 might run at 108MHz

[NIIBE Yutaka]

Hello,

I learned about Air32F103, another clone of STM32F103.  Unfortunately,
there is no reference manual available yet.

Air32F103:
https://wiki.luatos.com/chips/air32f103/index.html

Datasheet (in Chinese) *is* available here:
https://wiki.luatos.com/chips/air32f103/hardware.html

I looked around the code at:
https://gitee.com/iosetting/air32f103-template/blob/master/Libraries/AIR32F10xLib/inc/air32f10x_rcc.h

Because we can see a constant RCC_USBCLKSource_PLLCLK_4Div5, I guess
that it could run at 108MHz with 0 wait cycle, USB enabled.

If we don't care about side channel signal (for flash accelerator),
perhaps, it could run at 216MHz.

Currently, for Gnuk, GD32F103 (which is used for FST-01SZ) runs at 96MHz
with 0 wait cycle, USB enabled.  (It can run at 108MHz, but not with
USB.)
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