mips: ath79: Fix ar71xx_regs.h indent

The indent in this file triggers my OCD, so fix it. Replace multiple
spaces with tabs and align the values in one column.
Signed-off-by: NMarek Vasut <marex@denx.de>
Cc: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>
Cc: Wills Wang <wills.wang@live.com>