ARC: HSDK: Fixup DW SDIO CIU frequency to 50000000Hz

DW SDIO controller has external CIU clock divider controlled via
register in the SDIO IP. Due to its unexpected default value
(we expected it to divide by 1 but in reality it divides by 8)
SDIO IP uses wrong CIU clock (it should be 100000000Hz but actual
is 12500000Hz) and works unstable (see STAR 9001204800).

So increase SDIO CIU frequency from actual 12500000Hz to 50000000Hz
by switching from the default divisor value (div-by-8) to the
minimum possible value of the divisor (div-by-2) in HSDK platform
code.

Signed-off-by: Eugeniy Paltsev <Eugeniy.Paltsev@synopsys.com>
Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>

board/synopsys/hsdk/hsdk.c

@@ -26,6 +26,10 @@ int board_early_init_f(void)
 	return 0;
 }
 
+#define SDIO_BASE              (ARC_PERIPHERAL_BASE + 0xA000)
+#define SDIO_UHS_REG_EXT       (SDIO_BASE + 0x108)
+#define SDIO_UHS_REG_EXT_DIV_2 (2 << 30)
+
 int board_mmc_init(bd_t *bis)
 {
 	struct dwmci_host *host = NULL;
@@ -36,12 +40,18 @@ int board_mmc_init(bd_t *bis)
 		return 1;
 	}
 
+	/*
+	 * Switch SDIO external ciu clock divider from default div-by-8 to
+	 * minimum possible div-by-2.
+	 */
+	writel(SDIO_UHS_REG_EXT_DIV_2, (void __iomem *) SDIO_UHS_REG_EXT);
+
 	memset(host, 0, sizeof(struct dwmci_host));
 	host->name = "Synopsys Mobile storage";
 	host->ioaddr = (void *)ARC_DWMMC_BASE;
 	host->buswidth = 4;
 	host->dev_index = 0;
-	host->bus_hz = 100000000;
+	host->bus_hz = 50000000;
 
 	add_dwmci(host, host->bus_hz / 2, 400000);