ARM: keystone2: configs: Correct burn_uboot_sp erase size

The NOR flash on Keystone 2 evms has a u-boot-spl partition size of
0x80000.

Currently burn_uboot_spi will erase 0x100000 from the spi NOR which will
cause a partial erase of the misc partition.

Fix this by correcting the erase size.

Signed-off-by: Franklin S Cooper Jr <fcooper@ti.com>