mailbox: arm_mhuv2: Fix sparse warnings

This patch fixes a bunch of sparse warnings in the newly added arm_mhuv2
driver.

drivers/mailbox/arm_mhuv2.c:506:24: warning: incorrect type in argument 1 (different address spaces)
drivers/mailbox/arm_mhuv2.c:506:24:    expected void const volatile [noderef] __iomem *addr
drivers/mailbox/arm_mhuv2.c:506:24:    got unsigned int [usertype] *
drivers/mailbox/arm_mhuv2.c:547:42: warning: incorrect type in argument 2 (different address spaces)
drivers/mailbox/arm_mhuv2.c:547:42:    expected unsigned int [usertype] *reg
drivers/mailbox/arm_mhuv2.c:547:42:    got unsigned int [noderef] __iomem *
drivers/mailbox/arm_mhuv2.c:625:42: warning: incorrect type in argument 2 (different address spaces)
drivers/mailbox/arm_mhuv2.c:625:42:    expected unsigned int [usertype] *reg
drivers/mailbox/arm_mhuv2.c:625:42:    got unsigned int [noderef] __iomem *
drivers/mailbox/arm_mhuv2.c:972:24: warning: dereference of noderef expression
drivers/mailbox/arm_mhuv2.c:973:22: warning: dereference of noderef expression
drivers/mailbox/arm_mhuv2.c:993:25: warning: dereference of noderef expression
drivers/mailbox/arm_mhuv2.c:1026:24: warning: dereference of noderef expression
drivers/mailbox/arm_mhuv2.c:1027:22: warning: dereference of noderef expression
drivers/mailbox/arm_mhuv2.c:1048:17: warning: dereference of noderef expression

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>