soc-family.inc: Add a default SOC_FAMILY value

soc-family.inc: Add a default SOC_FAMILY value

Otherwise, if MACHINEOVERRIDES is expanded before SOC_FAMILY is set
(which may happen as MACHINEOVERRIDES is included in OVERRIDES) we can
see:

ExpansionError: Failure expanding variable MACHINEOVERRIDES, expression was
${@['', '${SOC_FAMILY}:']['${SOC_FAMILY}' != '']}p1022ds
which triggered exception SyntaxError: EOL while scanning string literal (MACHINEOVERRIDES, line 1)

To avoid this, give SOC_FAMILY a default empty value so it doesn't
get read as None.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>