mb/google/soraka: Do not reset PMIC during sleep

1. Due to reset signal, PMIC loses its internal register state. This
   causes PMIC to be in improper state after sleep.
2. The intent of reset signal is to reset internal state of PMIC (which
   happens once during power on), hence avoid asserting reset signal
   when not needed.
3. As per PMIC (TPS68470) datasheet, device can be kept in SLEEP mode
   when not in use to save max possible power.

To fix the same, do not reset PMIC while entering sleep.

By keeping PMIC in SLEEP mode, Power consumption is < 1uW (Typ) upto
3.63uW (Max). Refs: TPS68470 datasheet.
Measured value: 0.66uW

TEST= Build the firmware for Soraka & boot to OS. Do S3 resume & check
whether PMIC internal registers state are preserved.

Change-Id: I93ce4d76b0376b64ae6d1067aca0fd7467af3582
Signed-off-by: Naresh G Solanki <naresh.solanki@intel.com>
Reviewed-on: https://review.coreboot.org/20264
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Furquan Shaikh <furquan@google.com>

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