spi: altera: Add short note about EPCS/EPCQx1

spi: altera: Add short note about EPCS/EPCQx1

Add short documentation-alike note on how to use the Altera SPI
driver with the EPCS/EPCQx1 FPGA IP block on SoCFPGA Cyclone V
into doc/SPI/README.altera_spi

Signed-off-by: Marek Vasut <[email protected]>
Cc: Chin Liang See <[email protected]>
Cc: Dinh Nguyen <[email protected]>
Cc: Albert Aribaud <[email protected]>
Cc: Pavel Machek <[email protected]>
Cc: Jagannadha Sutradharudu Teki <[email protected]>
Acked-by: Pavel Machek <[email protected]>
Reviewed-by: Jagannadha Sutradharudu Teki <[email protected]>