ARM: zynq: 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
ps7_init() earlier before the first access to UART.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Link: https://lore.kernel.org/r/18e67e02a0c7190839a1ef3a11f3fd6babcf34cc.1645104518.git.michal.simek@xilinx.com