src/jtag/drivers/ep93xx: fix GCC 12 warning

src/jtag/drivers/ep93xx: fix GCC 12 warning

New GCC reports 5 warning:

src/jtag/drivers/ep93xx.c: In function 'set_gonk_mode':
src/jtag/drivers/ep93xx.c:123:47: warning: pointer of type 'void *' used in arithmetic [-Wpointer-arith]
  123 |         devicecfg = *((volatile int *)(syscon + 0x80));
      |                                               ^
src/jtag/drivers/ep93xx.c:124:35: warning: pointer of type 'void *' used in arithmetic [-Wpointer-arith]
  124 |         *((volatile int *)(syscon + 0xc0)) = 0xaa;
      |                                   ^
src/jtag/drivers/ep93xx.c:125:35: warning: pointer of type 'void *' used in arithmetic [-Wpointer-arith]
  125 |         *((volatile int *)(syscon + 0x80)) = devicecfg | 0x08000000;
      |                                   ^
src/jtag/drivers/ep93xx.c: In function 'ep93xx_init':
src/jtag/drivers/ep93xx.c:182:46: warning: pointer of type 'void *' used in arithmetic [-Wpointer-arith]
  182 |         gpio_data_register = gpio_controller + 0x08;
      |                                              ^
src/jtag/drivers/ep93xx.c:183:56: warning: pointer of type 'void *' used in arithmetic [-Wpointer-arith]
  183 |         gpio_data_direction_register = gpio_controller + 0x18;
      |                                                        ^

Change pointer type to allow pointer arithmetic.

Change-Id: Idd78a7156bdf99df2624043e924b8e54a0588ace
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/7180
Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>