libpayload: Add minimal support for PL011 UART

This creates a new PL011 config variable which avoids the
infinite busy wait on serial_putchar() because the register
mapping is not compatible with current implementation.

BUG=None
BRANCH=none
TEST=printf() works on the PL011 based ARMv8 foundation model

Change-Id: I9feda35a50a3488fc504d1561444161e0889deda
Signed-off-by: Marcelo Povoa <marcelogp@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/187020
Reviewed-by: David Hendricks <dhendrix@chromium.org>

payloads/libpayload/Config.in

@@ -168,6 +168,11 @@ config 8250_SERIAL_CONSOLE
 	default y if ARCH_X86
 	default n if !ARCH_X86
 
+config PL011_SERIAL_CONSOLE
+	bool "PL011 compatible serial port driver"
+	depends on 8250_SERIAL_CONSOLE
+	default n
+
 config SERIAL_IOBASE
 	## This default is currently not used on non-x86 systems.
 	hex "Default I/O base for the serial port (default 0x3f8)"

payloads/libpayload/drivers/serial.c

@@ -63,6 +63,7 @@ static void serial_hardware_init(int speed, int word_bits,
 {
 	unsigned char reg;
 
+#ifndef PL011_SERIAL_CONSOLE
 	/* Disable interrupts. */
 	serial_write_reg(0, 0x01);
 
@@ -81,6 +82,7 @@ static void serial_hardware_init(int speed, int word_bits,
 	/* Restore the previous value of the divisor.
 	 * And set 8 bits per character */
 	serial_write_reg((reg & ~0x80) | 3, 0x03);
+#endif
 }
 #endif
 
@@ -136,7 +138,9 @@ void serial_putchar(unsigned int c)
 {
 	if (!serial_hardware_is_present)
 		return;
+#ifndef CONFIG_LP_PL011_SERIAL_CONSOLE
 	while ((serial_read_reg(0x05) & 0x20) == 0) ;
+#endif
 	serial_write_reg(c, 0x00);
 }