linux-nuvoton: Add everspin RoT mailbox backoff

Platforms building linux-nuvoton do not inherit linux-gbmc.inc and
therefore miss the everspin RoT mailbox backoff logic.

Add a dynamic-layer linux-nuvoton_%.bbappend under nuvoton-layer in
meta-gbmc-staging to splice everspin.c into the kernel source tree during
do_patch, ensuring all Nuvoton builds receive the RoT mailbox backoff
routines. Scope FILESEXTRAPATHS specifically to drivers/mtd/spi-nor to
avoid pulling in any unrelated files from linux-gbmc.

Tested: Built a 5.10 platform image and ran on the platform. Verified that erase commands use nearly no CPU time even though they wait.

Fusion-Link: https://fusion2.corp.google.com/fc3599f8-4be9-3183-ba46-17749bf5c840 (platform11)
Fusion-Link: https://fusion2.corp.google.com/54e16a6d-e2bd-30b1-a175-e001028f694f (platform11-emr)
Fusion-Link: https://fusion2.corp.google.com/8034cdb4-eb8d-35e6-b05b-4c475dd6795c (platform15)
Fusion-Link: https://fusion2.corp.google.com/556cc957-9ada-35bd-b33e-9ba375015929 (platform17)
Fusion-Link: https://fusion2.corp.google.com/4a6bd3e1-e2a7-3987-9ff0-6544a7a7f1a5 (platform5)
Google-Bug-Id: 565629335
Change-Id: I76339ae4633948058361318546e5e10571f57c44
Signed-off-by: William A. Kennington III <wak@google.com>