Update Paul Burton's email address

MIPS is no longer a part of Imagination Technologies, and as such my
@imgtec.com email address will soon cease to function. This patch
updates occurrances of it with my new @mips.com email address, and adds
an entry in .mailmap such that git (& tools such as get_maintainer.pl
when examining history) will use the new address.

Signed-off-by: Paul Burton <paul.burton@mips.com>
Cc: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>
Cc: u-boot@lists.denx.de