platform/x86: Allow for SMBIOS backend defaults

Avoid accidental configurations by setting default y for DELL_SMBIOS
backends. Avoid this impacting the default build size, by making them
dependent on DELL_SMBIOS, so they only appear when DELL_SMBIOS is
manually selected, or by DELL_LAPTOP or DELL_WMI.

While DELL_SMBIOS does have a prompt, it does not have any dependencies.
Keeping DELL_SMBIOS visible, despite being "select"ed by DELL_LAPTOP and
DELL_WMI, is a deliberate choice to provide context for the WMI and SMM
backends, which would otherwise appear to float without context within
the menu.

Signed-off-by: Darren Hart (VMware) <dvhart@infradead.org>