mb/system76: Increase size of SMMSTORE to 512KB

[crawfxrd]

Match what was done for Google and Star Labs boards and increase the region size from 256K to 512K.

The previous default size of 256KB provided for only 64KB of actual
space for EFI variables, and after accounting for fragmentation, did
not provide enough free space for applying updates, such as for the
UEFI revocation database (DBX). Increasing it to 512KB allows for
192KB space for variables, and allows the UEFI DBX to be updated
properly via fwupd.

• Upstream: https://review.coreboot.org/c/coreboot/+/91768
• Ref: https://review.coreboot.org/c/coreboot/+/90287