ANDROID: arm64: configs: enabe CONFIG_TMPFS

I was not able to boot my QEMU with android-4.19-stable branch.
linux, android-5.4 and android-mainline are fine.

The root cause is that CONFIG_TMPFS is removed from defconfig
by the commit eb1e6716 ("arm64: defconfig: sync with savedefconfig")
and commit e4411602 ("ANDROID: drm/msm: Remove Kconfig default").

I sometimes need to run android-4.19-stable with defconfig on QEMU
to do some tests or bisect issues. Hopefully we can have
CONFIG_TMPFS=y back in defconfig.

Crash log:

[    4.089784] Run /init as init process
mount: mounting none on /run failed: Invalid argument
[    4.199358] Kernel panic - not syncing: Attempted to kill init! exitcode=0x0000ff00
[    4.199358]
[    4.200078] CPU: 7 PID: 1 Comm: init Not tainted 4.19.130-00652-g4f2c5aef95b4 #1
[    4.200172] Hardware name: linux,dummy-virt (DT)
[    4.200359] Call trace:
[    4.200467]  dump_backtrace+0x0/0x15c
[    4.200567]  show_stack+0x14/0x1c
[    4.200648]  dump_stack+0xb8/0xf0
[    4.200701]  panic+0x12c/0x2b4
[    4.200744]  exit_mm+0x0/0x1b8
[    4.200787]  __arm64_sys_exit_group+0x0/0x10
[    4.200839]  __wake_up_parent+0x0/0x28
[    4.200887]  __se_sys_exit_group+0x0/0x10
[    4.200940]  el0_svc_common+0x98/0x150
[    4.200987]  el0_svc_handler+0x5c/0x64
[    4.201035]  el0_svc+0x8/0xc
[    4.201376] SMP: stopping secondary CPUs
[    4.216220] Kernel Offset: disabled
[    4.216417] CPU features: 0x00000000,20006082
[    4.216497] Memory Limit: none
[    4.217005] ---[ end Kernel panic - not syncing: Attempted to kill init! exitcode=0x0000ff00
[    4.217005]  ]---

Fixes: e4411602 ("ANDROID: drm/msm: Remove Kconfig default")
Signed-off-by: Miles Chen <miles.chen@mediatek.com>
Change-Id: Iad9612bd1589e7de4d83808806acb94874da61e6