Allow microcontroller.nvm to be bigger than 64k

[hmaerki]

Fix

This PR allows NVM to be bigger than 64kBytes.

Bug

Assume mpconfigboard.h specifies: #define CIRCUITPY_INTERNAL_NVM_SIZE (136*1024).

But len(microcontroller.nvm) will return 8192 as the uint16_t wraps.

# Wrap uint16_t (wrong)
>>> (136*1024)%(2**16)
8192
# No wrap uin32_t (correct, this PR)
>>> (136*1024)%(2**32)
139264

With this PR applied, len(microcontroller.nvm) will return 139264 which is, as expected, 136*1024.

Test

I tested this PR on a nordic pca10059 using #define CIRCUITPY_INTERNAL_NVM_SIZE (136*1024).

[tannewt]

Thank you!