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

stand/efi/loader/copy.c

@@ -178,6 +178,8 @@ efi_verify_staging_size(unsigned long *nr_pages)
 #ifndef EFI_STAGING_SIZE
 #if defined(__amd64__)
 #define	EFI_STAGING_SIZE	100
+#elif defined(__arm__)
+#define	EFI_STAGING_SIZE	32
 #else
 #define	EFI_STAGING_SIZE	64
 #endif