Fix RP2350 boards with PSRAM crashing after raising the CPU speed

[mikeysklar]

What

PSRAM clock divider and read delay now follow the CPU speed.

Why

Setting microcontroller.cpu.frequency above 150 MHz corrupted PSRAM reads, then crashed into safe mode.

import microcontroller
microcontroller.cpu.frequency = 250_000_000
b = bytearray(4 * 1024 * 1024)  # lands in PSRAM

Uses the arduino-pico rule: SCK at or under 133 MHz, read delay matched to the divider. Follows up #11250.

Hardware tested

Fruit Jam (8 MB PSRAM)

How I tested it

Cold boot, set the clock, then write and verify 4 MB of PSRAM:

Clock	Before	After
150 MHz	OK	OK
200 MHz	MemoryError	OK
216 to 300 MHz	Hard fault, safe mode	OK

AI assistance

Claude Code helped find, fix and test this on my Fruit Jam.

[dhalbert]

Thanks!