tty: serial: amba-pl011: Mark 'sbsa_uart_acpi_match' as __maybe_unused

tty: serial: amba-pl011: Mark 'sbsa_uart_acpi_match' as __maybe_unused

When !ACPI 'sbsa_uart_acpi_match' is not referenced.

Fixes the following W=1 kernel build warning(s):

 drivers/tty/serial/amba-pl011.c:2792:36: warning: ‘sbsa_uart_acpi_match’ defined but not used [-Wunused-const-variable=]

Cc: Russell King <linux@armlinux.org.uk>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Jiri Slaby <jirislaby@kernel.org>
Cc: linux-serial@vger.kernel.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/20201104193549.4026187-18-lee.jones@linaro.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>