MLK-16930 imx6: hab: Fix secure boot build error and warning

MLK-16930 imx6: hab: Fix secure boot build error and warning

Found secure boot build error and warning for some i.MX6 platforms without
CONFIG_ROM_UNIFIED_SECTIONS defined. Fix them in this patch.

In file included from arch/arm/imx-common/hab.c:14:0:
arch/arm/imx-common/hab.c: In function ‘hab_rvt_report_event’:
./arch/arm/include/asm/imx-common/hab.h:152:59: error: ‘HAB_RVT_BASE_ARM64’ undeclared (first use in this function)
^
arch/arm/imx-common/hab.c:23:29: note: in expansion of macro ‘HAB_RVT_REPORT_EVENT_ARM64’
((hab_rvt_report_event_t *)HAB_RVT_REPORT_EVENT_ARM64) : \
^
arch/arm/imx-common/hab.c:153:30: note: in expansion of macro ‘hab_rvt_report_event_p’
hab_rvt_report_event_func = hab_rvt_report_event_p;
^
./arch/arm/include/asm/imx-common/hab.h:152:59: note: each undeclared identifier is reported only once for each function it appears in
^
arch/arm/imx-common/hab.c:23:29: note: in expansion of macro ‘HAB_RVT_REPORT_EVENT_ARM64’
((hab_rvt_report_event_t *)HAB_RVT_REPORT_EVENT_ARM64) : \

arch/arm/imx-common/hab.c:126:23: warning: 'gd_save' defined but not used [-Wunused-variable]
static volatile gd_t *gd_save;

Signed-off-by: Ye Li <ye.li@nxp.com>
Acked-by: Peng Fan <peng.fan@nxp.com>