env: Enable SPI flash env for SiFive FU540

SPI flash device on HiFive Unleashed has 32MiB Size.

This patch adds SPI flash environment after U-Boot proper
partition with a size of 128KiB.

SPI flash partition layout(32MiB):
    0 - 34	: reserved for GPT header
   35 - 39	: unused
   40 - 2087	: loader1 (SPL, FSBL)
 2088 - 10279	: loader2 (U-Boot proper, U-Boot)
10280 - 10535	: environment
10536 - 65494	: rootfs
65528 - 65536	: distro script

Note: the loader1 must start from 40th sector even though
there are 6 free sectors prior since 40th sector is nearest
flash sector boundary. 

Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Reviewed-by: Bin Meng <bin.meng@windriver.com>
Tested-by: Bin Meng <bin.meng@windriver.com>