Fix unstable temperature on HC32

[shadow578]

Description

When using MPCTEMP on HC32-based boards and setting the hotend temperature high (240C), strong oscillations of the hotend temperature occur.

While i'm not quite sure what causes this, i've found that reducing the ADC's operating frequency (PCLK2) to 25MHz eliminates the issue.

Other findings:

• enabling hardware averaging of the ADC channels dampens the oscillations slightly
• increasing or decreasing the sampling times does not affect oscillations
• using HAL_ADC_RESOLUTION of 12 bit while PCLK2=50MHz makes the oscillations stronger than 10 bit, but with PCLK2=25MHz 12 bit yields slightly more stable temperatures
• with 8-bit adc, marlin fails to boot

All testing was done over ~5 Minutes at 240C, followed by ~5 Minutes at 220C, with a fresh MPC autotune before each test.

See Fig. A and Fig. B for before and after.

Requirements

HC32-based mainboard with MPCTEMP enabled.

Benefits

Makes hotend temperature much more stable.

Configurations

N/A

Related Issues

N/A

Screenshots

Fig. A: Hotend temperature oscillations at 240C, on current bugfix-2.1.x branch	Fig. B: Stable hotend temperatures on this PR's branch