otPlatSpiSlavePrepareTransaction() function

Prepare data for the next SPI transaction. Data pointers MUST remain valid until the transaction complete callback is called by the SPI slave driver, or until after the next call to `otPlatSpiSlavePrepareTransaction()`. May be called more than once before the SPI master initiates the transaction. Each *successful* call to this function will cause the previous values from earlier calls to be discarded. Not calling this function after a completed transaction is the same as if this function was previously called with both buffer lengths set to zero and `aRequestTransactionFlag` set to `false`. Once `aOutputBufLen` bytes of `aOutputBuf` has been clocked out, the MISO pin shall be set high until the master finishes the SPI transaction. This is the functional equivalent of padding the end of `aOutputBuf` with `0xFF` bytes out to the length of the transaction. Once `aInputBufLen` bytes of aInputBuf have been clocked in from MOSI, all subsequent values from the MOSI pin are ignored until the SPI master finishes the transaction. Note that even if `aInputBufLen` or `aOutputBufLen` (or both) are exhausted before the SPI master finishes a transaction, the ongoing size of the transaction must still be kept track of to be passed to the transaction complete callback. For example, if `aInputBufLen` is equal to 10 and `aOutputBufLen` equal to 20 and the SPI master clocks out 30 bytes, the value 30 is passed to the transaction complete callback. If a `NULL` pointer is passed in as `aOutputBuf` or `aInputBuf` it means that that buffer pointer should not change from its previous/current value. In this case, the corresponding length argument should be ignored. For example, `otPlatSpiSlavePrepareTransaction(NULL, 0, aInputBuf, aInputLen, false)` changes the input buffer pointer and its length but keeps the output buffer pointer same as before. Any call to this function while a transaction is in progress will cause all of the arguments to be ignored and the return value to be `OT_ERROR_BUSY`.