MAINTAINERS: Add Purism mail alias as reviewer for their devkit's panel

Add a mail alias as reviewer for the rocktech jh057n00900 panel driver.

Signed-off-by: Guido Günther <agx@sigxcpu.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/a392758c914d436ec5e449980f619bc906c3054a.1561542477.git.agx@sigxcpu.org