arm64: zynqmp: Fix misspelled choice default

There is no JTAG symbol in the "Boot mode" choice. JTAG_MODE was
probably intended.

No functional changes. Kconfig choices fall back on using the first
(visible) symbol in the choice as the default if the default symbol is
not visible.

Discovered in Kconfiglib (https://github.com/ulfalizer/Kconfiglib),
which prints the following warning:

	warning: the default selection JTAG (undefined) of <choice> (defined at arch/arm/cpu/armv8/zynqmp/Kconfig:107) is not contained in the choice

I've added a corresponding warning to the C tools too, which is
currently in linux-next: https://patchwork.kernel.org/patch/9983667/



Signed-off-by: Ulf Magnusson <ulfalizer@gmail.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>