[IA64] fix PAL call in physical mode.

[IA64] fix PAL call in physical mode.

Now firmware code is mapped at its own virtual address space
which is same to Linux. So the calculation from virutal
address to physical address should be same to the linux one.
In practice firmware is located in low physical address area
so that masking 60 bit instead of 61 bits doesn't make difference.

I just checked and with the patch below,
xen/arch/ia64/linux-xen/pal.S becomes the same as
arch/ia64/kernel/pal.S from linux 2.6.21.

Signed-off-by: Simon Horman <horms@verge.net.au>