arch: arm: clear r7 to match aarch64 smp implementation

[juickar]

add mov r7, #0 as aarch64 implementation uses wzr which returns 0 on read but r7 might have an undefined value after a reset.

https://github.com/zephyrproject-rtos/zephyr/blob/main/arch/arm/core/cortex_a_r/reset.S :

    /* some core already won, release */
    mov r7, #0
    strb r7, [r4, r2]
    b _secondary_core

    /* suggest current core then release */
1:  str r1, [r0, #BOOT_PARAM_MPID_OFFSET]
+   mov r7, #0
    strb r7, [r4, r2]
    dmb

https://github.com/zephyrproject-rtos/zephyr/blob/main/arch/arm64/core/reset.S :

	/* some core already won, release */
	strb	wzr, [x4, x2]
	b	secondary_core

	/* suggest current core then release */
1:	str	x1, [x0, #BOOT_PARAM_MPID_OFFSET]
	strb	wzr, [x4, x2]
	dmb	ish

[sonarqubecloud]

Quality Gate passed

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