Reduce size of EFI_STAGING_SIZE to 32 on arm

Reduce the size of the EFI_STAGING area we allocate on arm to 32. On arm SBC
such as the NanoPi-NEOLTS the staging area allocation will fail on the 256MB
model with a staging size of 64.

Reviewed by: bcran, manu
Approved by: bz (mentor)
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D21016