MTD/CFI: flash_read64 is defined a weak function (for SPARC)

SPARC has implemented __raw_readq, it reads 64-bit from any 32-bit address.
SPARC CPUs implement flash_read64 which calls __raw_readq.

For current SPARC architectures (LEON2 and LEON3) each read from the
FLASH must lead to a cache miss. This is because FLASH can not be set
non-cacheable since program code resides there, and alternatively disabling
cache is poor from performance view, or doing a cache flush between each
read is even poorer.

Forcing a cache miss on a SPARC is done by a special instruction "lda" -
load alternative space, the alternative space number (ASI) is processor
implementation spcific and can be found by including <asm/processor.h>.
Signed-off-by: NDaniel Hellstrom <daniel@gaisler.com>

drivers/mtd/cfi_flash.c

@@ -239,12 +239,14 @@ static u32 flash_read32(void *addr)
 	return __raw_readl(addr);
 }
 
-static u64 flash_read64(void *addr)
+static u64 __flash_read64(void *addr)
 {
 	/* No architectures currently implement __raw_readq() */
 	return *(volatile u64 *)addr;
 }
 
+u64 flash_read64(void *addr)__attribute__((weak, alias("__flash_read64")));
+
 /*-----------------------------------------------------------------------
  */
 #if defined(CFG_ENV_IS_IN_FLASH) || defined(CFG_ENV_ADDR_REDUND) || (CFG_MONITOR_BASE >= CFG_FLASH_BASE)