microblaze: Improve failure handling for GPIO reset

Early exit from of_platform_reset_gpio_probe() if there
was no GPIO reset line configured.

Avoid kernel oops in gpio_system_reset():

[   27.413294] Restarting system.
[   27.415674] Machine restart...
[   27.418787] Oops: kernel access of bad area, sig: 11
[   27.423252]  Registers dump: mode=83871D1C
[   27.427428]  r1=00000000, r2=00000000, r3=FFFFFEF8, r4=00000000
[   27.433310]  r5=C026AED0, r6=00000001, r7=00000068, r8=00000000
[   27.439189]  r9=C3871DAC, r10=000011A5, r11=00000000, r12=0000000A
[   27.445318]  r13=00000000, r14=0000000F, r15=C00029BC, r16=00000000
[   27.451558]  r17=C011DE8C, r18=80000115, r19=0000000F, r20=48184ED8
[   27.457770]  r21=00000000, r22=FFFFFFEA, r23=00000001, r24=FEE1DEAD
[   27.463982]  r25=00000054, r26=1000B1C8, r27=00000000, r28=00000000
[   27.470208]  r29=00000000, r30=00000000, r31=C32D30C0, rPC=C011DE8C
[   27.476433]  msr=000042A2, ear=0000004B, esr=00000872, fsr=342E3732

And remove useless dump_stack from machine_restart.

Signed-off-by: Stephan Linz <linz@li-pro.net>
Signed-off-by: Michal Simek <monstr@monstr.eu>