Set cpu frequency based on F_CPU / board_build.f_cpu

[mh-dm]

Set cpu frequency based on F_CPU / platformio board_build.f_cpu. Fixes the potential pitfall where someone might try using board_build.f_cpu resulting in an F_CPU that differs from the one actually configured (through LPC_SC->PLL0CFG). F_CPU is used in system/lpc176x/time.h for the delay* functions and is also widely used in projects like Marlin.

Only allows 120mhz on MCU_LPC1769 and only the most commonly configured frequencies.

[mh-dm]

Comments welcome, including on whether I should remove the 96mhz option.

[p3p]

I'm reluctant to merge this without the runtime check for LPC1769 to avoid Marlin users overclocking LPC1768 boards accidentally

[mh-dm]

The check in #elif F_CPU == 120000000 && MCU_LPC1769 works to prevent an accidental overclock. Or are you talking about a scenario where Marlin users accidentally configure for an LPC1769 board when actually using an LPC1768 board?

[p3p]

indeed, there is no check at flash time (via sdcard) that the mcu is what the binary was built for, the original isLPC1769() does it at runtime which will make sure the clock is correct at the cost of the bad timing constant.

[mh-dm]

Good news then: Marlin has checks to make sure the selected architecture incl. the microcontroller matches the board. Example that I just tried:
Error: Build environment 'LPC1769' is incompatible with BOARD_BTT_SKR_V1_3. Use one of these environments: LPC1768

This prevents an accidental overclock like from setting default_envs = LPC1769 (which sets MCU_1769) in platform.ini but when actually using a 1768 board.