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>

arch/arm/cpu/armv8/zynqmp/Kconfig

@@ -113,7 +113,7 @@ config SPL_ZYNQMP_ALT_BOOTMODE
 choice
 	prompt "Boot mode"
 	depends on SPL_ZYNQMP_ALT_BOOTMODE_ENABLED
-	default JTAG
+	default JTAG_MODE
 
 config JTAG_MODE
 	bool "JTAG_MODE"