add SPI1 pins for BeagleBone Black

Refer to:
http://beagleboard.org/static/images/cape-headers-spi.png

CE1 P9.28 SPI1_CS0
MISO_1 P9.29 SPI1_D0
MOSI_1 P9.30 SPI1_D1
SCLK_1 P9.31 SPI_SCLK

SPI1 conflicts with HDMI Audio (McASP)

Refer to:
https://elinux.org/Beagleboard:BeagleBoneBlack_Debian#U-Boot_Overlays

To Disable HDMI AUDIO, uncomment this line in /boot/uEnv.txt:
disable_uboot_overlay_audio=1

Set pin modes for SPI1 with:

config-pin p9.28 spi_cs
config-pin p9.29 spi
config-pin p9.30 spi
config-pin p9.31 spi_sclk