hwmon: (xgene) access mailbox as RAM

The newly added hwmon driver fails to build in an allmodconfig
kernel:

      ERROR: "memblock_is_memory" [drivers/hwmon/xgene-hwmon.ko] undefined!

According to comments in the code, the mailbox is a shared memory region,
not a set of MMIO registers, so we should use memremap() for mapping it
instead of ioremap or acpi_os_ioremap, and pointer dereferences instead
of readl/writel.

The driver already uses plain kernel pointers, so it's a bit unusual
to work with functions that operate on __iomem pointers, and this
fixes that part too.

I'm using READ_ONCE/WRITE_ONCE here to keep the existing behavior
regarding the ordering of the accesses from the CPU, but note that
there are no barriers (also unchanged from before).

I'm also keeping the endianness behavior, though I'm unsure whether
the message data was supposed to be in LE32 format in the first
place, it's possible this was meant to be interpreted as a byte
stream instead.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Hoan Tran <hotran@apm.com>
Tested-by: Hoan Tran <hotran@apm.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>