MAINTAINERS: Add Ulf Hansson to the MEMORYSTICK section

The amount of changes to the memorystick subsystem are limited as of today.
However, I have a couple of times been funneling changes through my MMC
tree and it have turned out fine. So, I am here by volunteering to continue
doing this, by adding myself and the link to the MMC tree to the MEMSTICK
section.

Cc: Alex Dubov <oakad@yahoo.com>
Cc: Alex Dubov <alex.dubov@gmail.com>
Cc: Maxim Levitsky <maximlevitsky@gmail.com>
Cc: Kai-Heng Feng <kai.heng.feng@canonical.com>
Cc: Rui Feng <rui_feng@realsil.com.cn>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Acked-by: Maxim Levitsky <maximlevitsky@gmail.com>