Regmap Tx8 - heapless buffer writing for SPI and I2C

[soypat]

Adds Device8Txer to the regmap package, providing transaction-based register access for I2C and SPI devices using pre-allocated buffers. This enables heapless multi-byte register writes by reusing fixed buffers across transactions, eliminating heap allocations during bus operations. The Device8Txer wraps Device8 and manages write/read buffer state through a transaction API:

• Tx() initiates a transaction with a register address
• AddWriteByte()/AddWriteData() stage bytes into the write buffer
• DoTxI2C()/DoTxSPI() execute the transaction over the respective bus

// Initialization.
var dtx Device8Txer
dtx.SetTxBuffers(make([]byte, 256), make([]byte, 256))

// Usage.
const (
	defaultAddr = 65
	REG_WRITE   = 0x1f
	IOCTL_CALL  = 0xc0
)
tx, err := dtx.Tx(REG_WRITE)
if err != nil {
	panic(err)
}
err = tx.AddWriteData(IOCTL_CALL, 0x80, 0x80)
if err != nil {
	panic(err)
}
var bus drivers.I2C
readData, err := tx.DoTxI2C(bus, defaultAddr, 20)
if err != nil {
	panic(err)
}
fmt.Println(readData)

[soypat]

@ysoldak @deadprogram check this out! Saw the pains in latest si5351 driver and came up with this!