xen/arm: Add Amlogic Meson SoCs earlyprintk support

This patch adds earlyprintk support for Amlogic Meson SoC based
boards.

ATF[1] and U-boot[2] already initialize the UART for us. So no need to do it again.

Tested With:
 http://wiki.friendlyarm.com/wiki/index.php/NanoPi_K2

[1]: https://github.com/ARM-software/arm-trusted-firmware/blob/master/drivers/meson/console/aarch64/meson_console.S#L92
[2]: https://github.com/u-boot/u-boot/blob/master/drivers/serial/serial_meson.c#L44

Signed-off-by: Amit Singh Tomar <amittomer25@gmail.com>

xen/arch/arm/arm64/debug-meson.inc

@@ -0,0 +1,55 @@
+/*
+ * xen/arch/arm/arm64/debug-meson.inc
+ *
+ * MESON specific debug code.
+ *
+ * Copyright (c) 2019, Amit Singh Tomar <amittomer25@gmail.com>.
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms and conditions of the GNU General Public
+ * License, version 2, as published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public
+ * License along with this program; If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#define AML_UART_WFIFO_REG      0x00
+#define AML_UART_STATUS_REG     0x0c
+
+#define AML_UART_TX_FIFO_FULL   21
+
+.macro early_uart_init xb, c
+/* Uart has already been initialized by Firmware, for instance by TF-A */
+.endm
+
+/*
+ * MESON UART wait UART to be ready to transmit
+ * xb: register which contains the UART base address
+ * c: scratch register
+ */
+.macro early_uart_ready xb c
+1:
+        ldr     w\c, [\xb, #AML_UART_STATUS_REG]        /* status register */
+        tbnz    w\c, #AML_UART_TX_FIFO_FULL, 1b         /* Check TXFIFO FULL bit */
+.endm
+
+/*
+ * MESON UART transmit character
+ * xb: register which contains the UART base address
+ * wt: register which contains the character to transmit
+ */
+.macro early_uart_transmit xb wt
+        str    \wt, [\xb, #AML_UART_WFIFO_REG]
+.endm
+
+/*
+ * Local variables:
+ * mode: ASM
+ * indent-tabs-mode: nil
+ * End:
+ */