Revise the value of return value of flash_get_page_size()
Revise the value of return value of flash_get_page_size()

Currently the return value of flash_get_page_size() is 1 since the min size per one writing is 1 byte by Flash spec.
However, I noticed that this value causes a enormous write time When writing large data such as FW update.
So I revised this value to 8 byte by considering the writing time, memory alignment and memory hole.

c1728p9/mbed-os,c1728p9/mbed-os,kjbracey-arm/mbed,andcor02/mbed-os,andcor02/mbed-os,andcor02/mbed-os,mbedmicro/mbed,mbedmicro/mbed,andcor02/mbed-os,mbedmicro/mbed,c1728p9/mbed-os,mbedmicro/mbed,c1728p9/mbed-os,mbedmicro/mbed,kjbracey-arm/mbed,c1728p9/mbed-os,kjbracey-arm/mbed,andcor02/mbed-os,kjbracey-arm/mbed,c1728p9/mbed-os,andcor02/mbed-os