arm64: zynqmp: Fix debug uart initialization

The commit 0dba4586 ("arm: Init the debug UART") calls
debug_uart_init() from crt0.S but it won't work because SOC is not
configured yet. That's why create board_debug_uart_init() which calls
psu_init() via new psu_uboot_init() earlier before the first access to UART
in SPL. In full U-Boot call psu_uboot_init() only when
CONFIG_ZYNQMP_PSU_INIT_ENABLED is enabled.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Link: https://lore.kernel.org/r/878dc2daaa8685346f889989fbfb98b2e44da7fb.1645104518.git.michal.simek@xilinx.com