arm: separate async stuff from main uart

arm/uart.h

@@ -3,17 +3,8 @@
 #include <span>
 #include <string_view>
 
-#include "async.h"
-#include "buffer.h"
-
 void InitUarts();
 
-async::task<buffer> UartRead(int size);
-async::task<> UartWrite(std::span<const std::byte> data);
-inline async::task<> UartWrite(std::string_view s) {
-    co_await UartWrite(std::as_bytes(std::span{s.data(), s.size()}));
-}
-
 // block until the provided buffer is full
 void UartReadBlocking(std::span<std::byte> data);
 inline uint8_t UartReadByteBlocking() {