serial: zynq: Remove sparse warnings

Warnings:
drivers/serial/serial_zynq.c:181:1: warning: symbol 'uart_zynq0_init' was not declared. Should it be static?
drivers/serial/serial_zynq.c:181:1: warning: symbol 'uart_zynq0_setbrg' was not declared. Should it be static?
drivers/serial/serial_zynq.c:181:1: warning: symbol 'uart_zynq0_getc' was not declared. Should it be static?
drivers/serial/serial_zynq.c:181:1: warning: symbol 'uart_zynq0_tstc' was not declared. Should it be static?
drivers/serial/serial_zynq.c:181:1: warning: symbol 'uart_zynq0_putc' was not declared. Should it be static?
drivers/serial/serial_zynq.c:181:1: warning: symbol 'uart_zynq0_puts' was not declared. Should it be static?
drivers/serial/serial_zynq.c:182:22: warning: symbol 'uart_zynq_serial0_device' was not declared. Should it be static?
drivers/serial/serial_zynq.c:184:1: warning: symbol 'uart_zynq1_init' was not declared. Should it be static?
drivers/serial/serial_zynq.c:184:1: warning: symbol 'uart_zynq1_setbrg' was not declared. Should it be static?
drivers/serial/serial_zynq.c:184:1: warning: symbol 'uart_zynq1_getc' was not declared. Should it be static?
drivers/serial/serial_zynq.c:184:1: warning: symbol 'uart_zynq1_tstc' was not declared. Should it be static?
drivers/serial/serial_zynq.c:184:1: warning: symbol 'uart_zynq1_putc' was not declared. Should it be static?
drivers/serial/serial_zynq.c:184:1: warning: symbol 'uart_zynq1_puts' was not declared. Should it be static?
drivers/serial/serial_zynq.c:185:22: warning: symbol 'uart_zynq_serial1_device' was not declared. Should it be static?

Signed-off-by: Michal Simek <michal.simek@xilinx.com>