QSPIFBlockDevice: fix misconception in minimum program size

Prior to this PR, the minimum program size (QSPI_MIN_PROG_SIZE) of
QSPIFBlockDevice was 256 by default and 512 for some targets.
Those values were in fact page sizes, not program sizes.

Here's an explanation:
* Most QSPI flashes can be programmed to a granularity of a
  single byte or a few bytes - no need to be a whole page.
  This should be the value of QSPI_MIN_PROG_SIZE. Applications
  need to align buffer sizes to this granularity when
  programming QSPI flashes.
* Each sending of the underlying QSPI program signal requires
  destination bytes to be located within the same page.
  If a QSPIFBlockDevice::program() call crosses page boundaries,
  this function breaks down the operation into multiple chunks,
  so it's not a concern for the application.

So this PR changes the default program size to 1 (byte), and
for targets with a 4-byte (1-word) read size it overrides the
program size.

Note: No config is needed for the page size, as it comes from
the SFDP table parsed during initialisation.

storage/blockdevice/COMPONENT_QSPIF/mbed_lib.json

@@ -18,7 +18,7 @@
         "QSPI_POLARITY_MODE": 0,
         "QSPI_FREQ": "40000000",
         "QSPI_MIN_READ_SIZE": "1",
-        "QSPI_MIN_PROG_SIZE": "256"
+        "QSPI_MIN_PROG_SIZE": "1"
     },
     "target_overrides": {
         "MX25R6435F": {
@@ -37,15 +37,18 @@
         "MCU_NRF52840": {
             "QSPI_FREQ": "32000000",
             "QSPI_MIN_READ_SIZE": "4",
-            "QSPI_MIN_PROG_SIZE": "256"
+            "QSPI_MIN_PROG_SIZE": "4"
         }, 
         "MCU_PSOC6": {
-            "QSPI_FREQ": "50000000",
-            "QSPI_MIN_PROG_SIZE": "512"
+            "QSPI_FREQ": "50000000"
         },
         "EFM32GG11_STK3701": {
             "QSPI_MIN_READ_SIZE": "4",
-            "QSPI_MIN_PROG_SIZE": "256"
+            "QSPI_MIN_PROG_SIZE": "4"
+        },
+        "MCU_LPC546XX": {
+            "QSPI_MIN_READ_SIZE": "4",
+            "QSPI_MIN_PROG_SIZE": "4"
         }
     }
 }