Fix OSPIFBlockDevice program size

From the datasheet of the only OctaSPI flash we currently support
(http://www.mt-system.ru/sites/default/files/docs/Macronix/mx25lm51245g_3v_512mb_v0.01.pdf):

After program/erase command is issued, auto program/erase
algorithms which program/erase and verify the specified page
or sector/block locations will be executed. Program command is
executed on byte basis, or page (256 bytes) basis, or word basis.
Erase command is executed on sector (4K-byte), or block (64K-byte),
or whole chip basis.

So the minimum write size is one byte.

Author: LDong-Arm

storage/blockdevice/COMPONENT_OSPIF/mbed_lib.json

@@ -23,7 +23,7 @@
         "OSPI_POLARITY_MODE": 0,
         "OSPI_FREQ": "40000000",
         "OSPI_MIN_READ_SIZE": "1",
-        "OSPI_MIN_PROG_SIZE": "256"
+        "OSPI_MIN_PROG_SIZE": "1"
     },
     "target_overrides": {
         "MX25LM51245G": {