arch/arm64: fix boot stage prints with CONFIG_ARCH_EARLY_PRINT=y

`boot_stage_puts` used by early asm calls arm64_lowputc() for each
character in a loop. During that loop it uses x1 as the pointer to
the next character to be printed. However, x1 is clobbered by
arm64_lowputc(), resulting in undefined behaviour (only the first
character of the string is guaranteed to be printed).

Fix this by using x19 instead.

Signed-off-by: George Poulios <gpoulios@census-labs.com>

arch/arm64/src/common/arm64_head.S

@@ -365,15 +365,16 @@ out:
  */
 
 boot_stage_puts:
-    stp   xzr, x30, [sp, #-16]!
+    stp   x19, x30, [sp, #-16]!
+    mov   x19, x1
 1:
-    ldrb  w0, [x1], #1       /* Load next char */
+    ldrb  w0, [x19], #1      /* Load next char */
     cmp   w0, 0
     beq   2f                 /* Exit on nul */
     bl    arm64_lowputc
     b     1b                 /* Loop */
 2:
-    ldp   xzr, x30, [sp], #16
+    ldp   x19, x30, [sp], #16
     ret
 
 .type boot_stage_puts, %function;