tree: Remove duplicated <arch/mmio.h>

<device/mmio.h> is supposed to chain-include <arch/mmio.h>. See
`Documentation/contributing/coding_style.md` section `Headers and includes`

Change-Id: I08f7480650b42df1613994146a026bd1e12dbf66
Signed-off-by: Elyes Haouas <ehaouas@noos.fr>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/82693
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Martin L Roth <gaumless@gmail.com>

src/soc/sifive/fu740/gpio.c

@@ -2,7 +2,6 @@
 
 #include <soc/addressmap.h>
 #include <stdint.h>
-#include <arch/mmio.h>
 #include <assert.h>
 #include <device/mmio.h>
 #include <gpio.h>

src/soc/sifive/fu740/sdram.c

@@ -5,7 +5,6 @@
 #ifndef _SIFIVE_SDRAM_H
 #define _SIFIVE_SDRAM_H
 
-#include <arch/mmio.h>
 #include <console/console.h>
 #include <device/mmio.h>
 #include <soc/addressmap.h>