mfd: db8500-prcmu: Add description for 'reset_reason' in kerneldoc

Each function parameter should be documented in kerneldoc format.

Squashes the following W=1 warnings:

 drivers/mfd/db8500-prcmu.c:2281: warning: Function parameter or member 'reset_code' not described in 'db8500_prcmu_system_reset'
 drivers/mfd/db8500-prcmu.c:3012: warning: Function parameter or member 'pdev' not described in 'db8500_prcmu_probe'

Cc: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>