sysreset: syscon: Don't assume default value for offset and mask property

Per the DT binding, <offset> is a required property. Let's abort
the probe if it is missing. For the <mask> property, current codes
assume a default value of zero, which is not correct either.

Signed-off-by: Bin Meng <bin.meng@windriver.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Pragnesh Patel <pragnesh.patel@sifive.com>