Let's use the same default frequency everywhere, for consistency.
It could be any frequency, but 4MHz is already used for other chips and
it seems like a reasonable frequency to me (not too fast for most chips
but still reasonably fast). Oh, and 4MHz is slow enough that it can be
inspected by a Saleae Logic 4 (that sadly has been discontinued).
Only some pins (notably including GPIO2 aka machine.LED) have GPIO for
the default function 1. Other pins (such as GPIO 15) had a different
function by default. Function 3 means GPIO for all the pins, so always
use that when configuring a pin to use as a GPIO pin.
In the future, the mux configuration will need to be updated for other
functions such as SPI, I2C, etc.
This is only very minimal support. More support (such as tinygo flash,
or peripheral access) should be added in later commits, to keep this one
focused.
Importantly, this commit changes the LLVM repo from llvm/llvm-project to
tinygo-org/llvm-project. This provides a little bit of versioning in
case something changes in the Espressif fork. If we want to upgrade to
LLVM 11 it's easy to switch back to llvm/llvm-project until Espressif
has updated their fork.
