[MTD] Remove gratuitous inclusion of ARM-only header from physmap.c

The physmap platform driver conversion added to physmap.c an include
of asm/mach/flash.h which is 1) ARM-specific; and 2) isn't actually
necessary.  Remove it.

Signed-off-by: Lennert Buytenhek <buytenh@wantstofly.org>
Signed-off-by: David Woodhouse <dwmw2@infradead.org>