ARM: ux500: avoid warning in ux500_read_asicid

phys_addr_t may be 64 bit, which causes this harmless warning
in ux500_read_asicid:

arch/arm/mach-ux500/id.c: In function 'ux500_read_asicid':
arch/arm/include/asm/io.h:159:19: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
 #define IOMEM(x) ((void __force __iomem *)(x))
                   ^
arch/arm/mach-ux500/id.c:40:9: note: in expansion of macro 'readl'
  return readl(IOMEM(UX500_VIRT_ROM + (addr & 0xfff)));

We can solve this in a nicer way by making UX500_VIRT_ROM have a proper
type to start with and calculating the address using pointer arithmetic.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>