MAINTAINERS: add a more searchable string for the H8300 architecture.

Add a parenthesized string of "H8300" for more convenient searchability
in the MAINTAINERS file.

Signed-off-by: Robert P. J. Day <rpjday@crashcourse.ca>
Cc: Yoshinori Sato <ysato@users.sourceforge.jp>
Cc: Joe Perches <joe@perches.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>