Add basic SPI regmap support into the driver.

Previous patches unconver that ksz_spi_write() is always ever called
with len = 1, 2 or 4. We can thus drop the if (len > SPI_TX_BUF_LEN)
check and we can also drop the allocation of the txbuf which is part
of the driver data and wastes 256 bytes for no reason. Regmap covers
the whole thing now.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Andrew Lunn <andrew@lunn.ch>
Cc: Florian Fainelli <f.fainelli@gmail.com>
Cc: Tristram Ha <Tristram.Ha@microchip.com>
Cc: Woojung Huh <Woojung.Huh@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>