cr50: drop SRAM based NVMEM support
cr50: drop SRAM based NVMEM support

This is a purely clean up patch, there is no need to keep alternative
code around, flash storage is essential for TPM.

BRANCH=cr50, cr50-mp
BUG=none
TEST=generated symbol maps before and after this patch are the same
     for the Cr50 board.

Change-Id: I0ac37bdbf7a636a7ea0f412c6ad608e2262f75d7
Signed-off-by: Vadim Bendebury <5515d6d2d0829cbe0dd0dcf2094aaded06d58514@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/1450274
Reviewed-by: Mary Ruthven <81380f3403fed74252e92df3ed012a1819ec6296@chromium.org>

coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec