board: ti: ks2_evm: Over ride spl_get_load_buffer function

board: ti: ks2_evm: Over ride spl_get_load_buffer function

Currently k2 spi boot is broken as the image header
is getting copied to an invalid memory location

CONFIG_SYS_TEXT_BASE - sizeof (struct image_size)
which maps to 0xc000000 - 0x40 = 0xbffffc0 being a reserved
location.

We cannot change the CONFIG_SYS_TEXT_BASE address as the single
stage boots like UART boot will need the address to be 0xc000000
hence override the spl_get_load_buffer to have image_header
address as CONFIG_SYS_TEXT_BASE aka 0xc000000

Signed-off-by: Keerthy <j-keerthy@ti.com>