regulator: core: Fix size limit of supply_map

Now the debugfs file supply_map has a size limit PAGE_SIZE and the user
can not see the whole content of regulator_map_list when it is larger
than this limit.

This patch uses seq_file instead to make sure supply_map shows the full
information of regulator_map_list.

Signed-off-by: Haishan Zhou <zhssmail@gmail.com>
Signed-off-by: Mark Brown <broonie@kernel.org>