Documentation: arm: Replace use of virt_to_phys with __pa_symbol

All low-level PM/SMP code using virt_to_phys() should actually use
__pa_symbol() against kernel symbols.  Update the documentation to move
away from virt_to_phys().

Cfr. commit 6996cbb2372189f7 ("ARM: 8641/1: treewide: Replace uses of
virt_to_phys with __pa_symbol")

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/arm/firmware.txt

@@ -60,7 +60,7 @@ Example of using a firmware operation:
 
 	/* some platform code, e.g. SMP initialization */
 
-	__raw_writel(virt_to_phys(exynos4_secondary_startup),
+	__raw_writel(__pa_symbol(exynos4_secondary_startup),
 		CPU1_BOOT_REG);
 
 	/* Call Exynos specific smc call */