rp2/rp2_flash: Add MICROPY_HW_FLASH_MAX_FREQ to replace fixed max freq.HEADmaster

Assuming a 133MHz capable flash in 91cff8e4f10ea665c5f3f4b16d62c98d6ca22037
caused `rp2_flash_set_timing_internal` to set out of range dividers for
some boards (anything with value of 4 and flash that doesn't tolerate
higher speeds).

This affected (at least) the XIAO RP2350 board, making it non-bootable.

Since Pico SDK's `PICO_FLASH_SPI_CLKDIV` is entirely unreliable on a system
with a variable system clock (users can change it at runtime) then use it
only to work out a default `MICROPY_HW_FLASH_MAX_FREQ`.

This value can be overridden in board config.

Note that RP2350's default clock is 150MHz, RP2040's is 125MHz and it has
been certified at 200MHz so it's quite possible that
`PICO_FLASH_SPI_CLKDIV` is unreliable even at standard RP2 clocks.

(If flash timings are marginal then this can manifest as instability rather
than outright failure.)

Fixes issue #17375.

Signed-off-by: Phil Howard <github@gadgetoid.com>

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