cosmetic: reset: ast2500: Rename driver and configs

1. Rename AST2500 reset driver from ast2500-reset.c
   to reset-ast2500.c
2. Rename AST2500 reset kconfig option from AST2500_RESET
   to RESET_AST2500

Signed-off-by: Chia-Wei, Wang <chiawei_wang@aspeedtech.com>
Reviewed-by: Ryan Chen <ryan_chen@aspeedtech.com>