fix(plat/marvell/a3720/uart): fix configuring UART clock

When configuring the UART_BAUD_REG register, the function
console_a3700_core_init() currently only changes the baud divisor field,
leaving other fields to their previous value.

This is incorrect, because the baud divisor is computed with the
assumption that the parent clock rate is 25 MHz, and since the other
fields in this register configure the parent clock, which could have
been changed by U-Boot or Linux.

Fix this function to also configure the other fields so that the UART
parent clock is selected to be the xtal clock.

For example without this change TF-A prints only

    ERROR: a3700_system_off needs to be implemented

followed by garbage after plat_crash_console_init() is called.

After applying this change instead of garbage it also print crash info:

    PANIC at PC : 0x0000000004023800
Signed-off-by: Pali Rohár <pali@kernel.org>
Change-Id: I72f338355cc60d939b8bb978d9c7fdd576416b81

drivers/marvell/uart/a3700_console.S

@@ -49,12 +49,9 @@ func console_a3700_core_init
 	lsl	w2, w2, #4
 	add	w1, w1, w2, lsr #1
 	udiv	w2, w1, w2
-	and	w2, w2, #0x3ff
+	and	w2, w2, #0x3ff /* clear all other bits to use default clock */
 
-	ldr	w3, [x0, #UART_BAUD_REG]
-	bic	w3, w3, 0x3ff
-	orr	w3, w3, w2
-	str	w3, [x0, #UART_BAUD_REG]/* set baud rate divisor */
+	str	w2, [x0, #UART_BAUD_REG]/* set baud rate divisor */
 
 	/* Set UART to default 16X scheme */
 	mov	w3, #0