pinctrl: Fix build warning with ARM64

Following build warning appears when pinctrl-single is built for ARM64:

In file included from drivers/pinctrl/pinctrl-single.c:10:0:
drivers/pinctrl/pinctrl-single.c: In function ‘single_configure_pins’:
./arch/arm/include/asm/io.h:43:28: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
 #define __arch_getw(a)   (*(volatile unsigned short *)(a))

Fix this by using phys_addr_t for variable reg instead of u32

Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>