ARM: fix write*() I/O accessors

Commit 3c0659b5 "ARM: Avoid compiler optimization for readb, writeb
and friends." introduced I/O accessors with memory barriers.
Unfortunately the new write*() accessors introduced a bug:

The problem is that the argument "v" gets evaluated twice.  This
breaks code like used here (from "drivers/net/dnet.c"):

	for (i = 0; i < wrsz; i++)
		writel(*bufp++, &dnet->regs->TX_DATA_FIFO);

Use auxiliary variables to avoid such problems.

Signed-off-by: Wolfgang Denk <wd@denx.de>
Cc: Albert Aribaud <albert.aribaud@free.fr>
Cc: Alexander Holler <holler@ahsoftware.de>
Cc: Dirk Behme <dirk.behme@googlemail.com>