fix(rk3399): explicitly define the sys_sleep_flag_sram type

Recent GCC versions now do array-bounds checking which fails for
sys_sleep_flag_sram because the struct is larger than the 8-bytes
size that (void *) is

This variable is only used in one place as the struct,
so it can be defined with the struct type.

Resolves:
plat/rockchip/px30/drivers/pmu/pmu.c: In function 'rockchip_soc_sys_pwr_dm_suspend':
plat/rockchip/px30/drivers/pmu/pmu.c:977:23: error: array subscript 'struct psram_data_t[0]' is partly outside array bounds of 'void[8]' [-Werror=array-bounds]
  977 |         psram_boot_cfg->pm_flag &= ~PM_WARM_BOOT_BIT;

Change-Id: Ifbe42d11d0c7875f6cb23dc0b7ffb3f3f90c55a8
Signed-off-by: Scott Parlane <scott@parlanenz.com>
diff --git a/plat/rockchip/common/pmusram/cpus_on_fixed_addr.h b/plat/rockchip/common/pmusram/cpus_on_fixed_addr.h
index 34af29a..bcd2a7c 100644
--- a/plat/rockchip/common/pmusram/cpus_on_fixed_addr.h
+++ b/plat/rockchip/common/pmusram/cpus_on_fixed_addr.h
@@ -48,7 +48,7 @@
 CASSERT(__builtin_offsetof(struct psram_data_t, boot_mpidr) == PSRAM_DT_MPIDR,
 	assert_psram_dt_mpidr_offset_mistmatch);
 
-extern void *sys_sleep_flag_sram;
+extern struct psram_data_t sys_sleep_flag_sram;
 
 #endif  /* __ASSEMBLER__ */