reset: socfpga: rename membase ptr to modrst_base

The only member of this driver's priv struct is a pointer, which is
called 'membase'. However, since this driver handles multiple sub-
architectures, this is not the base address from dts but the base
address of some common registers of those sub-arches.

Reflect this better in sourcecode by renaming 'membase' to 'modrst_base'.

Signed-off-by: Simon Goldschmidt <simon.k.r.goldschmidt@gmail.com>