arm64: zynqmp: Also support JTAG as alternative boot mode

arm64: zynqmp: Also support JTAG as alternative boot mode

if (reg >> BOOT_MODE_ALT_SHIFT) condition rules out alternative jtag boot
mode which is 0. When 0 was used origin(HW) boot mode was used instead.
That's why directly fill reg variable with requested boot mode and don't
let code to read value back. "else" part of code remain unchanged.

Reviewed-by: Sean Anderson <sean.anderson@linux.dev>
Signed-off-by: Michal Simek <michal.simek@amd.com>
Link: https://lore.kernel.org/r/ed51a9d51948ab939a53e0b9dc6c2d2546f97a4f.1710933505.git.michal.simek@amd.com