spi: stm32-ospi: Increase read throughput in indirect mode

Schedule() is called every u8/u16 or u32 read accesses which is overkill.
Move schedule() from stm32_ospi_read_fifo() to stm32_ospi_tx_poll()
and call schedule() only every 1MB chunk of data.

Test performed by reading 64MB on sNOR on stm32mp257f-ev1 board:

          before      after    ratio
Read  :  10.6MB/s    14.2MB/s   +34%

Signed-off-by: Patrice Chotard <patrice.chotard@foss.st.com>
Change-Id: I5552e21bfd8d8a68a7148c9dbf96039a66e946db
Reviewed-on: https://gerrit.st.com/c/mpu/oe/st/u-boot/+/518998
ACI: CITOOLS <MDG-smet-aci-reviews@list.st.com>
Domain-Review: Christophe KERELLO <christophe.kerello@st.com>
ACI: CIBUILD <MDG-smet-aci-builds@list.st.com>

Author: Patrice Chotard

drivers/memory/stm32-omi.c

@@ -298,7 +298,6 @@ static void stm32_omi_read_fifo(void *val, phys_addr_t addr, u8 len)
 	case sizeof(u8):
 		*((u8 *)val) = readb_relaxed(addr);
 	};
-	schedule();
 }
 
 static void stm32_omi_write_fifo(void *val, phys_addr_t addr, u8 len)
@@ -358,6 +357,9 @@ int stm32_omi_tx_poll(struct udevice *dev, void *buf, u32 len, bool read)
 		fifo(buf, regs_base + OSPI_DR, step);
 		len -= step;
 		buf += step;
+
+		if (!(len % SZ_1M))
+			schedule();
 	}
 
 	return 0;