linux-user: Add support for read/clear RTC voltage low detector using ioctls

linux-user: Add support for read/clear RTC voltage low detector using ioctls

This patch implements functionalities of following ioctls:

RTC_VL_READ - Read voltage low detection information

    Read the voltage low for RTCs that support voltage low.
    The third ioctl's' argument points to an int in which
    the voltage low is returned.

RTC_VL_CLR - Clear voltage low information

    Clear the information about voltage low for RTCs that
    support voltage low. The third ioctl(2) argument is
    ignored.

Implementation notes:

    Since one ioctl has a pointer to 'int' as its third agrument,
    and another ioctl has NULL as its third argument, their
    implementation was straightforward.

Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Signed-off-by: Filip Bozuta <Filip.Bozuta@rt-rk.com>
Message-Id: <1579117007-7565-7-git-send-email-Filip.Bozuta@rt-rk.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>