risc-v/mpfs: fix compile warning

After commit b7d2b38, the system suffers from -Warray-bounds warning with -Wall:

chip/mpfs_opensbi.c: In function 'mpfs_hart_to_scratch':
chip/mpfs_opensbi.c:251:26: warning: array subscript hartid is outside array bounds of 'sbi_scratch_holder_t[0]' {aka 'struct sbi_scratch_holder_s[]'} [-Warray-bounds]
  251 |   return (unsigned long)(&g_scratches[hartid].scratch);

Fix it by reverting back to what is was earlier. g_scratches shouldn't be in the bss
region that would be zeroed out.

Signed-off-by: Eero Nurkkala <eero.nurkkala@offcode.fi>

arch/risc-v/src/mpfs/mpfs_opensbi.c

@@ -225,7 +225,7 @@ static const struct sbi_platform platform =
 
 /* This must go into l2_scratchpad region, starting at 0x0a000000. */
 
-sbi_scratch_holder_t g_scratches[0] \
+sbi_scratch_holder_t g_scratches[MPFS_MAX_NUM_HARTS] \
                __attribute__((section(".l2_scratchpad")));
 
 /****************************************************************************