ARM: keystone: Pass SPI MTD partition table via kernel command line

SPI U-Boot image for K2 boards have now exceeded 512K partition
allocated to it and no longer fit the partitions defined in kernel DTS
file. Therefore, pass an updated MTD partition table from U-Boot as
kernel command line arguments to avoid kernel from accidentally
modifying boot loader image that has overflowed to next user partition.

To do is, introduce a common environment file for declaring SPI
partition so that each individual boards need not repeat the same.
Choose appropriate SPI bus from board config file and pass it as command
line argument to kernel.

Signed-off-by: Vignesh R <vigneshr@ti.com>