drivers: spi: Add default char to mcux flexcomm spi driver

Adds optional device tree property to specify a default character
to clock out when the TX buffer pointer is NULL. If the property is
not set the existing behavior (default char of 0x00) is used.

I verified the expected behavior using an i.MX RT685 board and
logic analyzer that the def-char character is transmitted when
TX buffer pointer is NULL.

Signed-off-by: Bryce Wilkins <bryce.wilkins@gmail.com>