MAINTAINERS: update entry to thermal governors file name prefixing

Commit 0015d9a2 ("thermal/governors: Prefix all source files with
gov_") renamed power_allocator.c to gov_power_allocator.c in
./drivers/thermal amongst some other file renames, but missed to adjust
the MAINTAINERS entry.

Hence, ./scripts/get_maintainer.pl --self-test=patterns complains:

  warning: no file matches    F:    drivers/thermal/power_allocator.c

Update the file entry in MAINTAINERS to the new file name.

Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Acked-by: Amit Kucheria <amit.kucheria@linaro.org>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20200728045850.22661-1-lukas.bulwahn@gmail.com