RP2040: Bypass cache when reading flash memory directly

[mikee47]

Using normal XIP window forces reads through cache which will affect code execution speed.

[what-the-diff]

• The variable flashaddr is not used in the function.
• XIP_NOCACHE_NOALLOC_BASE + fromaddr should be assigned to xip\ctrl\hw->stream\addr instead of flashadd, because it's a physical address and can't be directly accessed by CPU.