mfd: sm501: dbg_regs attribute must be read-only

Fix:

sm501 sm501: SM501 At b3e00000: Version 050100a0, 8 Mb, IRQ 100
Attribute dbg_regs: write permission without 'store'
------------[ cut here ]------------
WARNING: at drivers/base/core.c:620

dbg_regs does not have a write function and must therefore be marked
as read-only.

Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Lee Jones <lee.jones@linaro.org>