uboot-lantiq: reduce stack size

uboot-lantiq: reduce stack size

On lantiq a lot of stuff expects to be loaded to and executed at
0x80002000, including our own second stage bootloader.

For all build u-boots, the initial stack pointer is at 0x80008000. After
loading data to 0x80002000, every further stack operation corrupts the
loaded code.

Set the initial stack pointer to 0x80002000, to not overwrite code
loaded in memory. A stack of 0x2000 bytes has been proven as enough in
all done tests.

Signed-off-by: Mathias Kresin <dev@kresin.me>