Fix #18 PWM frequency

[fpistm]

Get the correct timer freq depending on timer clock source.

When TIMPRE bit of the RCC_DCKCFGR register is reset,

• if APBx prescaler is 1, then TIMxCLK = PCLKx,
• otherwise TIMxCLK = 2x PCLKx.

When TIMPRE bit in the RCC_DCKCFGR register is set,

• if APBx prescaler is 1,2 or 4, then TIMxCLK =HCLK,
• otherwise TIMxCLK = 4x PCLKx

[fpistm]

Tests Fading/tone/Servo done on F0/F3/F4/F7/L4 -->ok

[ghost]

PWM pins tested on L0/L4/F0/F3/F4 -> OK

I found another issue on F0 & F3: some PWM pins are not configured in PeripheralPins.c.

• F0: D3=PB3 & D6=PB10 not set
• F3: D6=PB10 not set

@fpistm can you confirm me that is a mistake? If yes I will push the patch.

[fpistm]

Hi,
this is not really a mistake.
This only means by default PWM is not enabled on those pins but they could be added.
In fact, I have took as example mbed PeripheralPins.c for those boards that's why it is commented.

[ghost]

It is OK for me.