drivers: mipi_dbi_spi: add 16-bit transfer to C4

Extends the MIPI DBI SPI driver class for operating mode C4, SPI 4-wire,
with 16 write clocks to send one or multiple byte for commands. Generic
data (e.g. GRAM) aligned to 16-bit are passed through and stuffed with
bytes if required.

Signed-off-by: Stephan Linz <linz@li-pro.net>