tests: flash: erase_blocks: Add README file

Add the brief README file for the erase_blocks test.
Every test/sample should have a readme file.

Signed-off-by: Andrej Butok <[email protected]>

Author: butok

tests/drivers/flash/erase_blocks/README.rst

@@ -0,0 +1,6 @@
+Flash Controller Erase Block Testing
+####################################
+
+Testing of erasing and writing all pages of the tested partition.
+If MCUBOOT is not selected, the partition under test is slot1_partition.
+If the test is built with MCUBOOT, the tested partition is boot_partition.